Reduced physics checks since 5.1?

I have a 2D platformer that I've been working as a side project for a while now, and even though I get some easier to fix issues in all the updates, 5.1 seems to have broken the physics and I can't get any solid results.

First of all the problem really only happens when my player is using a skill named 'fastfall' in which he is falling with a downwards force of 10k+. He goes through the terrain, but only sometimes, which made me assume that sometimes the physics is updated at the correct time so that he doesn't fall through, and sometimes the physics check is a millisecond or less too late, which ends up making him fall through. Any idea for sure? Am I correct in my assumption or is there anything else in 5.1 that would break a 2D platformer dependent on split-second physics checks?