In the recent update, mobs that are together in a small hole or room will take damage if more than 24 of them are inside of each other. If a player or another mob adds to this amount, the mobs in question will take damage rapidly until one or more dies.
While this might be an intentional change, I wanted to be sure. The change has broken a few things in my world(s), so if the bug is fixed/the limit of mobs inside each other before damage is raised, it would be very much appreciated.

I looked a bit and found a similar report that a mod responded to with:
[Mod] Skylinerw added a comment - 17/Oct/16 11:53 AM
As of 1.11, mobs will take damage when occupying the same space. The threshold is specified by the maxEntityCramming gamerule (defaulting to 24).
Case closed!