SUMMARY
Installed application digiKam from Discover store. 
Go to settings - plugins 
Plugin list is empty. 

Environment
cat /etc/*release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04.3 LTS"

printf 'Desktop: %s\nSession: %s\n' "$XDG_CURRENT_DESKTOP" "$GDMSESSION"
Desktop: KDE
Session: plasma

STEPS TO REPRODUCE
1. Install ubuntu 
2. Install KDE environment
3. Open Discovery and install digiKam

OBSERVED RESULT

Plugin list is empty. 
EXPECTED RESULT

Plugin list should be populated with installable plugins like fliker, Dropbox etc.

Additional information 

I asked on Telegram Ubuntu group
Another user installed the application and confirmed they got the same result

I tried to install using Snap and Flatpak and both gave the same result.
Only way was to get an appimage which I got from here:
https://download.kde.org/stable/digikam/6.3.0/digikam-6.3.0-x86-64.appimage.mirrorlist

This app image the plugin list is working