My microphone does not work with FaceTime, but works everywhere else

In every other scenario, all of the microphones work perfectly on my 4th generation iPad Pro 12.9” except when I use FaceTime; callers are not able to hear me. This issue was not resolved at the Genius Bar in the Apple Store when they reinstalled the OS. Without explanation, they said it’s a hardware issue. How? I would appreciate that someone with computer engineering experience explains this claim.

[Re-Titled by Moderator]