I'm using ExCompressum 3.0.17 for 1.12.2, and noticed that occasionally on killing a compressed zombie (using a zombie apocalypse map, so have not tested on compressed skeletons, creepers, etc.), they don't always split into 9 zombies. Wonder why this happens, or if it was made this way to not be unfair to the player.

On a separate, slightly unrelated note, why no name tag drop anymore? Is it purely to help with potential lag from spawning so many random zombies?

I checked again and this was a bug after all, compressed mobs currently stop splitting if the world was reloaded after they spawned.

Compressed Mobs only split if killed directly by a player, so if they die through any other means (traps, fire, poison, wither, certain AOE weapons) they will not split. This was done to prevent the splitting from messing with mobspawners in skyblocks.

Name tag drops were never a feature, they were caused by some other mod that was falsely assuming that an entity with a name must have had a nametag used on it. I assume that was fixed or removed.

Understood, however there were a few that I have killed directly that did not split. I dealt them the final blow with a sweeping edge 1 Sharpness III diamond sword. Is the damage caused by a sweep attack technically AoE and thus would not cause the split? Huh.