minecraft:is_stackable causes entities to still clip

Summary:
minecraft:is_stackable causes entities to still clip through each other when added to entities behaviour. You should normally see with minecraft:is_stackable is that the models of the entities should collide and not clip through each other.

Repro Steps:

1. Create a world with the attached world Stackable Test.mcworld
2. Summon 2 Zombie Horses
3. Then using one of the horse try to collide with the other horse
4. Will not collide the first few times but then will stop and clip through the model

Expected Results:
Should never collide through each other similar to boats and minecarts.

Screenshots/Videos attached:
Yes. [Stackable Collide Bug|https://www.youtube.com/watch?v=ywPFw797jd4]