Mobs have been easily forgetting the player and never seem to remember them or where they are whenever they go out of their sight.

Mobs that I encountered so far that has this issue: Zombie, Skeleton, Creeper

Repro steps from comments below: 
How to reproduce:

1. Go about 40 blocks into the air, and create a little zig-zag walk way out of full blocks. On the corners place glass pillars.
2. summon a creeper on one end of the walk way, make it target you, and then walk behind one of the glass pillars.
3. You will see the creeper instantly stop, forget about you completely, and just stand there until it decides to randomly pathfind.