The bug

When a left-handed enderman attacks a player, they will swing their left arm differently compared to right-handed endermen.

To reproduce

1. Summon an enderman

    /summon minecraft:enderman ~ ~ ~ {Attributes:[{Base:-10.0d,Name:"generic.movement_speed"},{Base:0.0d,Name:"generic.attack_damage"}],LeftHanded:1b} 

2. Either attack it or look at it in the eyes
   → It's swings it's left arm in a weird way

Video

Minecraft 1.16.1 - Singleplayer 2020-07-11 22-24-18.mp4