maybe it's mistaking multiple voices as background noise and cancelling them out, resulting in muting/filtering.
just to be sure, could you take the phone to UBreakIFix and have them checkout the MIC?
Is it happening only in Speaker mode Calls with multiple people? if so, it's possible that it's not a hardware issue... rather it could be something software related that just needs tweeking to compensate for multiple people speaking at once.
Reproduce the issue whenever you are able to and then submit an error report to Samsung Members app to see if they can do anything about it. Try to be descriptive and clear about the issue, it seems the issue can be easily misunderstood as something else