microphone not working for facetime video calls

i have a brand new iphone 12 mini everything works fine except the microphone when on facetime video calls and whatsapp video calls, surprisingly the microphone works fine when on a regular call or facetime audio call, i took it to the authorized service center they ran a diagnostic test, and seems like there is no issue at all either with software or hardware, they placed a facetime video call to test it, they were able to detect that there was a issue with the microphone, but unfortunately they could not help me because their so called software did not detect and issue. now i am stuck with a brand new iphone 12 mini with a non working facetime, extremely disappointed, also going thru lot of forums online it seems like this is a very common problem.

kindly help if anyone facing same issue and their problem has been resloved

i dont have a screen protector on my phone

i am not using any kind of phone cover either

software version is 15.0.2

did a factory reset

restored it

nothing works