not 100% sure if this is an glitch just something that should be checked out ; )

Shot an enderman with an poison arrow I expected the enderman to teleport away and not get affected.

Shot the enderman and it teleported then I found the same enderman with a poison effect. I thought it was a coincidence, so I started to shot more than they all got affected. I later found out when, when I shot an enderman at a wall the enderman got affected again and the arrow was in the wall.

Poison arrow (to see the effect)

Shot an arrow at an enderman

look around and you will see it got affected

now shot an enderman at an wall and look at the wall you will see the arrow there.