Ive seen this ALOT. Hold your Thumb on the trigger hand down with your thumb from the brace hand right on the thumbnail.
So if you are shooting Right handed hold down your right thumb with the left thumb. Shouldn't affect your grip or accuracy on the 9c compact.
Because of the shorter grip, people tend to try to get high enough on it to maybe slide in a pinky or get a better grip. After shooting a few they loosen the grip on the recoil and the thumb hits it.
If you ever have to use a firearm, you don't get to pick the scenario!