When I have updates for both Flatpak and PackageKit apps available, when I click the "check for updates" button, the list of PackageKit updates briefly disappears, leaving only the Flatpak apps. Then in a moment they all re-appear. This is somewhat awkward. I've attached a screen recording from 5.15.5, though I can reproduce on 5.16 too.
Created attachment 120816 [details] Screen recording depicting the problem
Git commit 67b313bdd6472e79e3d500f8b32d0451c236ce84 by Aleix Pol. Committed on 14/06/2019 at 23:59. Pushed by apol into branch 'Plasma/5.16'. flatpak: Indicate that updates are being fetched M +2 -0 libdiscover/backends/FlatpakBackend/FlatpakBackend.cpp https://commits.kde.org/discover/67b313bdd6472e79e3d500f8b32d0451c236ce84
That commit didn't appear to fix the issue entirely. Flatpak apps now disappear during update operations, but Flatpak runtimes are still shown. I've attached a screen recording that shows it still happening with Discover from git master.
Created attachment 121488 [details] Still happening with git master
Seems fixed now in 5.18.