Looks like you are not the only one with this issue:
I’ll keep looking.
EDIT:
I’ll also add this one (also without solution, unfortunately).
I wonder if it’s a bug in Plasma6 which will be fixed? Couldn’t find any bug reports there mentioning this though.
I do see on this machine that audio defaults to the other one (via bluetooth) which currently isn’t working. It did, but I’ll find out if it does again when I run the updates on that machine later today.