SUMMARY
Discover and updater systray icon always show that flatpak apps are pending to update.

Systray shows that 16 packages are pending to update. When click on the "Show updates" button and Discover is launched, it shows thar 14 packages are pendind to update.

Command "flatpak update" shows no pending to update apps.

If Discover is launched from command-line, this is the output:

alex@forvo-laptop:~
└─ $ plasma-discover 
file:///usr/lib/x86_64-linux-gnu/qt5/qml/org/kde/kirigami.2/GlobalDrawer.qml:258: TypeError: Cannot read property 'length' of undefined
file:///usr/lib/x86_64-linux-gnu/qt5/qml/org/kde/kirigami.2/GlobalDrawer.qml:293: TypeError: Cannot read property 'length' of undefined
file:///usr/lib/x86_64-linux-gnu/qt5/qml/org/kde/kirigami.2/GlobalDrawer.qml:292: TypeError: Cannot read property 'length' of undefined
adding empty sources model QStandardItemModel(0x5646f2576850)
no packages for "im.riot.webapp"
invalid kns backend! "/etc/xdg/ksysguard.knsrc" because: "Config group not found! Check your KNS3 installation."
invalid kns backend! "/etc/xdg/servicemenu.knsrc" because: "Config group not found! Check your KNS3 installation."
org.kde.plasma.libdiscover: Couldn't find a category for  "fwupd-backend"
Couldn't get latest commit for com.skype.Client
Couldn't get latest commit for org.keepassxc.KeePassXC
Couldn't get latest commit for org.libreoffice.LibreOffice
Couldn't get latest commit for org.freedesktop.Platform.Icontheme.Adwaita
Couldn't get latest commit for org.freedesktop.Platform.Icontheme.Paper
Couldn't get latest commit for org.freedesktop.Platform.Locale
Couldn't get latest commit for org.freedesktop.Platform.VAAPI.Intel
Couldn't get latest commit for org.freedesktop.Platform.VAAPI.Intel
Couldn't get latest commit for org.freedesktop.Platform.ffmpeg
Couldn't get latest commit for org.freedesktop.Platform.html5-codecs
Couldn't get latest commit for org.freedesktop.Sdk.Locale
Couldn't get latest commit for org.gnome.Platform.Locale
Couldn't get latest commit for org.gnome.Platform.Locale
Couldn't get latest commit for org.gnome.Platform
Couldn't get latest commit for org.gnome.Sdk.Locale
Couldn't get latest commit for org.gnome.Sdk
Couldn't get latest commit for org.gtk.Gtk3theme.Breeze-Dark
Couldn't get latest commit for org.gtk.Gtk3theme.Breeze
Couldn't get latest commit for org.kde.KStyle.Adwaita
Couldn't get latest commit for org.kde.KStyle.Adwaita
Couldn't get latest commit for org.kde.Platform.Locale
Couldn't get latest commit for org.kde.Platform
Couldn't get latest commit for org.libreoffice.LibreOffice.Locale
Fwupd Error 9 Failed to find any useful results to report

If I try to update the apps, this is the output show in command-line:

Failed to install "org.libreoffice.LibreOffice" : "app/org.libreoffice.LibreOffice/x86_64/stable rama 885915922756348b18c47bf6fc4c2454055b51a678cb66da3937af16cdb7a5cd ya se encuentra instalada"
Failed to install "org.gtk.Gtk3theme.Breeze" : "runtime/org.gtk.Gtk3theme.Breeze/x86_64/3.22 rama 29ee9f32136b64260696874961abd5d0a07d943a0f90611348fe3d1345e01691 ya se encuentra instalada"
Failed to install "org.gnome.Platform" : "runtime/org.gnome.Platform/x86_64/3.26 rama 206dc8b94b672634830efc1203c0330fce77c9ddd0a823089f1f237476367d57 ya se encuentra instalada"
Failed to install "org.freedesktop.Platform.Icontheme.Paper" : "runtime/org.freedesktop.Platform.Icontheme.Paper/x86_64/1.0 rama aedae3896a65d123e2e1466573bb88c51ea983aad41f9dffd6b4d70c2de4f1d1 ya se encuentra instalada"
Failed to install "org.freedesktop.Platform.Icontheme.Adwaita" : "runtime/org.freedesktop.Platform.Icontheme.Adwaita/x86_64/1.0 rama d1caa338205100dcf62174502ed1cb40a1ddb47a5314aff9a10a1a4db0478bf0 ya se encuentra instalada"
Failed to install "org.kde.Platform" : "runtime/org.kde.Platform/x86_64/5.10 rama e55a6e84318584e2f75c1e9d080a154d665ba9e2b243f77033dda6086be30758 ya se encuentra instalada"
Failed to install "org.freedesktop.Platform.ffmpeg" : "runtime/org.freedesktop.Platform.ffmpeg/x86_64/1.6 rama d757f762489e444551d17223590635d4f4014a36b6fecd73323bf3f95f1aa490 ya se encuentra instalada"
Failed to install "org.gnome.Sdk" : "runtime/org.gnome.Sdk/x86_64/3.28 rama 70d9a1d6966acf73797ea555322422f62220b7bf5f938473d46c3565ad8e1518 ya se encuentra instalada"
Failed to install "org.freedesktop.Platform.VAAPI.Intel" : "runtime/org.freedesktop.Platform.VAAPI.Intel/x86_64/1.6 rama 82006efc71d3c9e122ee6845147920ef6fe4054d4978f8686120afc85e89f2b0 ya se encuentra instalada"
Failed to install "org.keepassxc.KeePassXC" : "app/org.keepassxc.KeePassXC/x86_64/stable rama 8702d16b321c4ed61f14f7e9dbbccea84cef01f369b474655f680277ddc79bb6 ya se encuentra instalada"
Failed to install "org.gtk.Gtk3theme.Breeze-Dark" : "runtime/org.gtk.Gtk3theme.Breeze-Dark/x86_64/3.22 rama 51d8c48571ec2f6e01ee615775475c37ab97da0efae8e4b307c9d177925fcdcc ya se encuentra instalada"
Failed to install "org.kde.KStyle.Adwaita" : "runtime/org.kde.KStyle.Adwaita/x86_64/5.10 rama 7056bef5f0be3ac2ec47eab5373dec7a787f3f260c9645661fb5144ba37c0237 ya se encuentra instalada"
Failed to install "com.skype.Client" : "app/com.skype.Client/x86_64/stable rama 6afe06fb827b484f682161c7c23eccaf8b6e4d0beb6da36eaff5c2d8351abe23 ya se encuentra instalada"
Failed to install "org.kde.KStyle.Adwaita" : "runtime/org.kde.KStyle.Adwaita/x86_64/5.9 rama 41f06d286c71bbcb622fcfec0910638abc71fadab514f93cb03efaa0f9bb60ad ya se encuentra instalada"

I think there is a problem with flatpak. Some weeks ago there was an issue with the flathub certicates and everytime that tryed to execute "flatpak update" that error was reported. Now that error is gone and "flatpak update" works fine.

I think the error is related to the missing last commit from apps.

If that helps for reproducing, here is the output of

alex@forvo-laptop:~
└─ $ flatpak list --ostree-verbose -v -d
F: No installations directory in /etc/flatpak/installations.d. Skipping
F: Opening user flatpak installation at path /home/alex/.local/share/flatpak
F: Opening system flatpak installation at path /var/lib/flatpak
Referencia                                            Origen  Commit activo Último commit Tamaño de la instalación Opciones      
com.google.AndroidStudio/x86_64/stable                flathub f61f48b1c6fe  -              421,9 kB                   system,current
com.skype.Client/x86_64/stable                        flathub 6afe06fb827b  ?               44,5 MB                   system,current
com.slack.Slack/x86_64/stable                         flathub af79c7981276  -                2,8 MB                   system,current
org.keepassxc.KeePassXC/x86_64/stable                 flathub 8702d16b321c  ?               13,3 MB                   system,current
org.libreoffice.LibreOffice/x86_64/stable             flathub 885915922756  ?              647,3 MB                   system,current
org.freedesktop.Platform.Icontheme.Adwaita/x86_64/1.0 flathub d1caa3382051  ?               27,5 MB                   system,runtime
org.freedesktop.Platform.Icontheme.Paper/x86_64/1.0   flathub aedae3896a65  ?               58,2 MB                   system,runtime
org.freedesktop.Platform.VAAPI.Intel/x86_64/1.6       flathub 82006efc71d3  ?                8,7 MB                   system,runtime
org.freedesktop.Platform.VAAPI.Intel/x86_64/18.08     flathub 11d9fc7d7bbd  ?                8,1 MB                   system,runtime
org.freedesktop.Platform.ffmpeg/x86_64/1.6            flathub d757f762489e  ?                7,7 MB                   system,runtime
org.freedesktop.Platform.html5-codecs/x86_64/18.08    flathub cb314686f1e0  ?                7,9 MB                   system,runtime
org.freedesktop.Platform/x86_64/1.6                   flathub 8b967508e3e7  -                1,1 GB                   system,runtime
org.freedesktop.Platform/x86_64/18.08                 flathub e8df934455b4  -              746,3 MB                   system,runtime
org.freedesktop.Sdk/x86_64/1.6                        flathub 41778b63ba68  -                1,9 GB                   system,runtime
org.gnome.Platform/x86_64/3.24                        flathub 72388eb9f5ab  -                1,3 GB                   system,runtime
org.gnome.Platform/x86_64/3.26                        flathub 206dc8b94b67  ?                1,3 GB                   system,runtime
org.gnome.Sdk.Docs/x86_64/3.30                        flathub 91d4beb6adf2  -              240,8 MB                   system,runtime
org.gnome.Sdk/x86_64/3.28                             flathub 70d9a1d6966a  ?                2,1 GB                   system,runtime
org.gnome.Sdk/x86_64/3.30                             flathub f4fb520fdd34  -                2,4 GB                   system,runtime
org.gtk.Gtk3theme.Breeze-Dark/x86_64/3.22             flathub 51d8c48571ec  ?              389,6 kB                   system,runtime
org.gtk.Gtk3theme.Breeze/x86_64/3.22                  flathub 29ee9f32136b  ?              387,1 kB                   system,runtime
org.kde.KStyle.Adwaita/x86_64/5.10                    flathub 7056bef5f0be  ?                1,4 MB                   system,runtime
org.kde.KStyle.Adwaita/x86_64/5.9                     flathub 41f06d286c71  ?                1,4 MB                   system,runtime
org.kde.Platform/x86_64/5.10                          flathub e55a6e843185  ?                1,6 GB                   system,runtime
org.kde.Platform/x86_64/5.9                           flathub 38e847d2af60  -                1,6 GB                   system,runtime


STEPS TO REPRODUCE
1. Start system
2. Click system tray icon 
3. Try to update apps via discover

OBSERVED RESULT
Disctint number of flatpak apps pendind to update in discover and updater systray icon. If apps are updated via discover, nothing changes and always remain shown as pending to update

EXPECTED RESULT
Discover updates flatpak apps and systray icon hides

SOFTWARE VERSIONS
(available in About System)
KDE Plasma Version: 5.14.1
KDE Frameworks Version: 5.51.0
Qt Version: 5.11.2

ADDITIONAL INFORMATION