Mobs sometimes fall through moving slime blocks

I built a flat with the ground hidden trapdoor, and I tested it and it worked fine. It just drops a block down from the floor then pulls it away and then launches me out with a slime block launcher, and quickly closes the floor so I land on top of it. I wanted to see what I looked like, so I did it twice with a zombie. the second time it fell through the slime block, but the weird thing was it was built in the sky, and I gave it fire resistance so it didn't move, but it just sank through the floor when it extended the slime block. I just built a different small launcher somewhere else, to see if it would happen with a cow (and to see if there was something it was hitting) and after around ten tries the cow fell through.