When I reboot my system, the default audio device is not selected correctly. This morning, my headset is set to default, instead of my desktop speakers. This has been a problem for a while now, but it is getting better. In the last version of KDE I was running, it didn't even show my desktop speakers after a reboot until I restarted pulseaudio. Now they show up, so that's good progress, but the default device after a reboot should be the same as before the reboot.

I'll try updating my pulseaudio config files manually. That worked last time, so I imagine it will work again, but it should be unnecessary.

Kubuntu 21.04
Plasma: 5.21.4
Frameworks: 5.80.0
QT: 5.15
Kernel 5.11.0-16-generic x64

I wonder if it is a typo that you have Frameworks 5.80.0 instead of 5.81.0...

Here's a screenshot of where I got that information.

Update: Setting my default sink in the pulse audio config file does work. It shouldn't be necessary to do that, though.

My bad, I enabled the backports PPA and forgot about.

As for the PA issue: you should file a bug report

Bug filed: https://bugs.kde.org/show_bug.cgi?id=436662

Thank you!