Another “interesting” phenomenon that seems to be associated with the change from pulseaudio to pipewire-pulse.
Whenever Audacious changes track and there’s a different number of channels in the new track (ie going from a mono track to a stereo one or vice-versa), the balance immediately jumps from centre to left.
Playing around with the output settings, I’ve found that changing the output plugin from PulseAudio to SDL seems to fix it, though I’ve no idea what the implications of making this change are.
[Edit]: curious. Certain (low-quality) podcasts end up with unpleasant artefacts when using SDL, so evidently that’s not going to be a good solution. I need to dig deeper.
[Edit 2]: as explained in this thread, pipewire is only actually required if the pulseeffects package is installed. So removing that and reinstalling the original pulseaudio solves all the problems.