Enderman was suffocating while in a boat and was being attacked by a tamed wolf. The enderman, although unmoving from the boat, was teleporting out of the boat, and was successful at attacking the player. 

What I expected to happen was...:
Enderman stayed in the boat while the wolf and player killed it.

What actually happened was...:
Enderman was suffocating while in a boat and was being attacked by a tamed wolf. The enderman, although unmoving from the boat, was teleporting out of the boat, and was successful at attacking the player. 

Steps to Reproduce:
1. Build a platform at 3 blocks high
2. Place boat on the edge, with a tamed wolf inside the boat
3. Have Enderman enter the boat so it's suffocating

Clip:

https://www.twitch.tv/mecaone1/clip/GlamorousGiantAlfalfaCoolCat-Y04URyFEr02rVLez

Full stream starting at 19:20

https://www.twitch.tv/videos/2215804280

Here is the same bug tested again on a different world:

https://youtu.be/oEDeMl2jYFo