Right airpod mic is not working when in ear

Here is the situation. My left airpod died. Apple

replaced it with new one. I had some troubles to pair it but eventually it

worked. After that, I noticed that the mic in right airpod does not work. So I

did some tests with Voice memo app. The result is: Mic works if airpod is out

of ear, as soon as I put it into my ear it stops recording my voice. Therefore,

I think the microphone is working properly. What else could be wrong? I mean

everything else works, the sound, ear detection, Siri, stop, play...

What could it be? Difference in

firmware, bug?

S.