[MC-71797] Sinking Mobs

I went to battle a couple of zombies in an artificial cave however they sank into the ground and pop up and down. It works for most mobs but not all.
Here is proof of my find: https://www.youtube.com/watch?v=T2vB_TOSLp4&feature=youtu.be

Linked issues

pls watch the video before commenting that it dosent happen to you

Misleading title would've made this a dupe of your own ticket, MC-71714, but dupe of MC-119

how is this a duplicate of floating spiders?