Why do calls occasionally sound terrible on the Pixel 3a?

I run LineageOS 19 on the Pixel 3a. Pretty regularly I'm told my call quality (microphone on my end) sounds awful. What might cause this? By using [this](https://android.stackexchange.com/questions/259322/how-can-i-force-a-recording-with-a-specific-microphone) , I think I isolated the issue. Recording in unprocessed 3gpp aac-lc mode sounds awful. I think software (including the phone/dialer?) is falling back to AAC-LC during stressful situations (for me, often when speaker is turned on?). I also get this issue in Discord calls on speakerphone. Enabling wi-fi calling with mint mobile seems to have helped though I still don't understand the root cause. It's not a hardware issue because the microphone sounds crystal clear in other settings of the same app.