iPhone XR - microphone issue ?

There ARE two microphones. If you bothered to RTFM (Read The Fine Manual) you would know that; even the first few pages of it. There is one at the bottom of the phone, and a 2nd one on the back near the camera that is used for noise cancellation. The back mic is only used when holding the phone on a call; it is not used in speakerphone mode or for other recording purposes. But the noise cancellation can be fooled if the back mic is partially covered by your hand, a case that doesn't fit, or a flip open folio case if you fold it most of the way back. If you are in the habit of using it with the back mic partially covered it will result in garbling, because it hears the same thing (your voice) in both mics and tries to cancel it. If you don't want the noise cancellation feature either cover the back mic with a piece of tape or turn off noise cancellation in Settings.