Well, as it says in the Support article Record the screen on your iPhone, iPad, or iPod touch - Apple Support, "Some apps might not allow you to record audio." Zoom might be one of those apps. (Zoom has its own internal recording mechanism, so they may prevent system recordings.) Are you noticing the problem in any other apps?
In a brief test on my iPad Pro (12.9" 5th gen) running iPadOS 15, screen recordings did capture from the microphone when I turned it on, but I only tried a couple of apps (one that allowed its own sound to be recorded, one that didn't).