The Bug
Mobs that use double layering for textures (such as drowneds) have lost their outer layer in the latest beta
Steps to reproduce
Spawn a drowned
Edit: So it seems that this was a texture pack related issue. The bug went away once I removed it. Seems like the person who ported it from the java edition didn't re-add the layers to the mobs.
I'll close it for you. 🙂