I have the same issue as you with 2 of the 3 32 rounders that I got earlier in March from Tactical Vantage. I finally got a chance to take out my new FDE MOE last Wednesday and the S&W FDE 25 round mag was fine. First ProMag would wiggle loose after 3 or 4 shots and drop out far enough to no longer feed. Re-inserted it and got a few more shots, rinse, repeat. Second ProMag was fine. Third had the same issue as the first. I was running CCI 36 grain bulk (Troy Landry Special Edition) through all the magazines.
I cleaned the rifle earlier tonight and had a chance to try the mags in just the lower. When I wiggle the 2 that would drop after several rounds they come loose.
Which part of the notch that the latch catches did you shave down? Did you make it deeper? Or taller? If taller, did you shave the upper or lower surface of the notch or both? How as it been working for you? I'm up for trying some minor tweaking but if it's going to be a major issue I'd rather just involve ProMag and send them back for service/replacement.