I don’t think it’s the mouth opening either. Do you have other woodies? The reason I ask is that there is a learning curve to tapping an iron in a wooden plane. In the beginning I had a hard time trying to nail the exact cutting depth and it was always too little or way too much. It takes a little practice to learn how to finesse it a half a thousandth of an inch.

Also check the bed to make sure it’s flat. If it’s convex the iron would rock when it catches the wood which would yield a nothing or way too much cut. If a previous owner tried to widen the mouth and didn’t know which side to file, I could see that happening. And just generally check for anomalies with the bed, both sides of the iron, and the back of the wedge.

If it's none of the above, recheck the sole to make sure it's really flat.