Normally I would agree with you. Negative pop is a real problem, even with much smaller enlargements. I've even seen it on 8x10" prints before I started to use glass carriers. However, I know for sure that it was not a problem here, since I used a glass negative carrier and once alignment was done, the problem was gone permanently. By the way, notice, I did not say "corners", but "edges" - left and right edges as a matter of fact .
My bad! Yes, edges and not corners. I'm a glass person also - be it scanning or enlarging!