Have been able to repeat multiple times in a single run with blob enemies that split apart when killed.
So far it has happened with railgun but wonder if it can happen to anything gun with sufficient knockback

blobs will attempt to die and split but if knocked into the wall too much they stay at 0 hp but don't die or split but still take damage and stop the wave from ending. 

i was using comrad in co-op at the time and it happened multiple times in the run and it was any wall that did this whether it was a pylon in middle of the room or a solid wall at the spawn of the blobs. was even able to use the shove enemy to heal of them. did not just happen to 1 enemy per floor i managed to get 10 of them since the blob spawn was in a small room and it was only the splitter enemies. It could also have something to do with also stunning them at the same time of the knockback into the wall with railgun.