SUMMARY I have a pair of headphones that the system did not automatically switch to. There was no button to set the default device in Plasma so I had to use pavucontrol to set the default device. SOFTWARE/OS VERSIONS Linux: Arch Linux KDE Plasma Version: 6.2.5 KDE Frameworks Version: 6.9.0 Qt Version: 6.8.1
Closing this. I'm unable to repro this again.
JFYI there is no such concept as "default device". When this happens, if you're using PulseAudio, it's generally caused by the "switch on connect" module not being loaded, which is a thing that can be changed in System Settings. if you're using PipeWire, it's an issue in Pipewire since PipeWire is supposed to have this behavior automatically.