System audio being output through headset microphone even if microphone is physically muted

Hello everyone.

I am having an issue with my system’s audio being transmitted through the microphone channel on my headset. This persists even if my mic is physically muted (flip-to-mute). This issue has been confirmed to happen across multiple headsets, and it can be reproduced no matter what recording software is used. I’ve tried googling what to do but apparently nobody has had this issue before (or worded it in the same way I have). I have absolutely no idea where to start on fixing this. Please help.