Hi, I experience the same problem with Bluetooth headphones.
My FP4 uses them, but not as microphone.
It uses the internal microphone. I have verified this by using the sound recording app: connect to the headphones and talk when the phone is close to me or far from me → the loudness changes, whereas for headphones’ mic it should not.
Similarly, the loudness changes when turning the phone.
That is a problem of FP4, and not of the headphones, as I tried this with both Fairphone Earbuds and another pair of headphones.
Further, my brother has tested it with his FP4 and his earbuds - same result.
It appears, thus, to be a software bug of FP4.
I’m on the last update: FP4.TP2L.C.0126.20240909.
I have acutely noticed the problem last week, but it might have been around for longer.
I’d appreciate it if someone else could test and confirm the issue independently. We could then tag the Fairphone developers to make them aware.