Yep, happens any time the AirPods mic is activated on MacOS, has been that way since launch as far as I know. Doesn't happen on iOS. I can only assume it has something to do with the Bluetooth hardware or they would have fixed it by now.
But it usually goes back once the mic is no longer being used. And I've had it happen a few times where I take a 'continuity phone call' from my iPhone on my Mac. The AirPods switch to the low-quality call mode, I have my conversation, I hang up, and the audio quality still stays switched until I do the Audio MIDI workaround or reboot.
Most of the time that doesn't happen, though—when I hang up the audio goes back to normal mode (48 kHz). So it seems to be a weird bug with the switching that I've only ever seen on the Mac (works perfect on iOS).
