skeletons and mobs not spawning right

skeletons spawn. then just stand there.

What I expected to happen was...:
shooting a bow at them they take a hit but after that arrows just bounce off and you can pic up your arrows again.

What actually happened was...:
skeletons mobs don't despawn or burn in daylight either

Steps to Reproduce:
1. use latest snapshot 15w51b or one like 15x47c. 15w47c or just before was when i first noticed it.
2. spawning may work for awhile but then skeletons not spawning right.
3. when the bug happens other spawns of mobs do not happen.

I included a short video of the problem. Also mobs now see holes if you try to use a sign to conceal the hole.