Almost certainly just a bug with closing the audio session. It doesn’t seem to always be listening but sometimes after a meeting it stays on for whatever reason. If it’s not already fixed then I’m sure it will be soon…
I can't see how it can be that hard to just close the mic device when you go off a call.
Another infamous example is proctoru. Literally a spyware, but delivering a spyware requires much less effort (both intellectually and financially) compared to designing a product that makes security-savy customers happy.