Ten minutes ago, I discovered a different problem while on a tech support call with Apple. I was addressing this issue and had my iPhone 13 Max on speaker. The tech said I was breaking up. He asked me if I had a case on the phone. I told him I had the Apple midnight leather MagSafe case. He asked me to take it off and try again. Sure enough, he said my voice was crystal clear. For whatever reason the case is interfering with the speaker phone. So he authorized a new case for me which I’ll pick up tomorrow from the Apple store.
This doesn’t address your question but I thought my situation is timely to your issue.