Looking at the date, I have to assume that this hasn't been solved yet. Which is beyond annoying and disappointing. This has been happening to my sister's iPhone 11 every time we talk for months if not a couple of years. Recently she got my mother her first iPhone and while we facetime video called together just now, it did the same thing! Other callers have pointed out the same issue when on a call with either of them.
If this hasn't already been said, the only thing that I've noticed, being on the other side, is that it only tends to happen when the receiver (a person without the iPhone 11) is talking. I have a 6s. While listening to her in the middle of a conversation, if I even do a "hmm..." or "right" or "uh-huh" or a short laugh or snort or sigh or any vocal sound- then her microphone cuts OUT, and I can't hear whatever it was she just said. Of course, this shouldn't happen.