Pipewire does not appear to handle fast user switching
If I am logged in as user A and I use fast user switching to switch to user B then sound does not work properly with pipewire seemingly because the pipewire instance for user A still has the devices open even though the ownership of the devices will have been changed as part of the user switch.
This seems to happen for at least whichever sound device is currently active for user A and also for bluetooth devices even if they're not actively in use.
Note that pulseaudio has never handled the bluetooth case but it did at least have per-user configuration so you could ensure that only one user had bluetooth enabled in pulseaudio. It did handle the builtin sound card though, releasing it from the first user so that the second user could use it.