SUMMARY
I System Settings -> Audio -> Applications I noticed that each of application placed on list has own ComboBox with list available audio devices. The issue is that nothing is selected there even sound is currently playing by one or two applications from the list. Please look at attached screen shot.

STEPS TO REPRODUCE
1. Start two application playing sound, for example webbrowser with running youtube service and any sound player
2. Switch audio output in section Devices in plasmoid "Audio volume". 
3. Go to System Settings -> Audio -> Applications
4. Look at ComboBoxes placed at right side which contain list of audio devices

OBSERVED RESULT
No audio device selected in ComboBox even application playing sound

EXPECTED RESULT
If sound is playing in headset then should be selected headset here

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: YES
(available in About System)
KDE Plasma Version: 5.20.0
KDE Frameworks Version: 5.75
Qt Version: 5.15.1

ADDITIONAL INFORMATION