Wither skeleton waving arms in the water when targeting a player

The bug

When in water and targeting a player, wither skeleton start to change its arm pose from 'aligned to the body like skeleton' to 'forward like a zombie', at rate +- 5 times a second.

How to reproduce

1. Spawn a wither skeleton
2. Hit it or punch it so it falls in the water
3. It starts waving its arms