Steps to reproduce

1. Trap an enderman in a boat.
2. Punch the enderman several times while it is in the boat.

Expected result

Each punch hurts the enderman.

Observed result

After one or a few punches you can no longer hit the enderman. If you use a repeating particle command to track it's position, you can see that it teleports away server-side (MCPE-31761) but remains in the boat visually.

After attacking an Enderman trapped on a boat several times, the attack has a certain probability of failure, and then you will not be able to attack the Enderman unless you destroy the boat, at which point the Enderman will immediately teleport.

Using hands to attack an Enderman full of blood will easily reproduce the problem.