This sounds less like a Google Meet issue and more like Orion not fully releasing the Bluetooth microphone session after the tab is closed. The quality drop during the meeting is expected because many Bluetooth headsets switch to a hands-free profile when the mic is active, but the audio should return to normal once the microphone is no longer being used.
A few things worth testing: after leaving Meet, check macOS Sound settings and see whether the headset microphone is still selected as the active input. You can also try switching the input temporarily to the MacBook’s built-in microphone while keeping the Bluetooth headset as output. That often prevents the headset from staying in the lower-quality call mode.
It may also help to remove microphone permission for Meet/Orion, restart the browser, and then grant access again. If the issue only happens in Orion and not Safari, Chrome, or Firefox, it’s probably a browser-level bug where the audio input stream is not being released properly.
For troubleshooting, I’d also do a quick mic check before and after the call using something simple like check microphone online to confirm whether the microphone is still being detected or held active.
Hopefully Orion can fix this in a future build because quitting the whole browser just to restore Bluetooth quality is definitely not ideal.