Looks like NixOS's pipewire service enables WirePlumber by default. Not sure if it changed recently from media-session, and that's why it's gone unstable, or if it's something else.
I've run into a similar issue, where the default output device wouldn't be selected, when I used PulseAudio by itself before Pipewire was a thing. I had to use one of the PA console mixers to set my default output and input devices, and with some playing around with it, I was able to make them the default. Might help to use pactl or one of the many PA mixers to poke around and see if there's anything you can adjust to fix the issue.
Might also help to test out pipewire-media-session to see if that helps, too.
When I was futzing around two or three months ago with audio on my NixOS system I believe it was still using media-session. Are you on unstable? I believe the intention was to migrate to WirePlumber, but didn't expect that to happen already.
Looks like NixOS's pipewire service enables WirePlumber by default. Not sure if it changed recently from media-session, and that's why it's gone unstable, or if it's something else.