The problem apears to be that the mag follower tab does not protrude out the back of the mag far enough, the follower wants to ride more forward in the mag body, and the end of the catch tab is not lifted upwards to active the catch.
So, even with squaring off the back of the mag, it's not going to solve the above problem, Hence the moded 9mm catch allows the catch tab to protrude more forward into the well to make sure that the follower tab presses the catch up, even when the follower wants to ride the front of the body upward.