With 1.13 an issue was fixed were mobs attempted to pathfind over 1.5 blocks high walls. MCPE-45256 But it hasn't fully been fixed. It still affects walls that are lower than 2 blocks and higher than 1.5 blocks.

How to reproduce:

1. Place a full block like for example stone.
2. Place a not one block high block that is not a slab above it. For example soul sand or an enchantment table or a bed. 
3. Try to lure a mob over the wall. For example lure a cow by holding wheat in your hand or just wait until it tries to do that itself. 

What I observed: The mob infinitely tries to jump over the wall.

What I expected: The mob doesn't try to pathfind over the wall. 
It does not work with 4 to 7 or more snow layers at one block, because all of them have a 0.5 blocks high collision box. (See MCPE-60024)