The bug

When you use /data modify entity on Endermen and use it to set their AngryAt data to the player's UUID it works, but when you run the exact same command but with a mob it doesn't aggro the Enderman onto that mob.

How to reproduce

1. Spawn a pig
2. Spawn an Enderman that is "AngryAt" the pig:

   /execute summon enderman run data modify entity @s AngryAt set from entity @e[type=pig,limit=1,sort=nearest] UUID

   → The Enderman doesn't attack the pig