Endermen seem to be immune to arrows, even when they can't teleport away. The arrows will just bounce off of them. To reproduce, spawn an enderman somewhere where it can't teleport away (e.g. a vast ocean, or a single block in the middle of an endless void) and shoot an arrow at it.
Thank you for your report!
However, this issue is a Duplicate of MC-79556.
That ticket has already been resolved, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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