The Bug
Signs render in front of the eyes of invisible endermen, even if the sign is much further back.
How to reproduce:
Spawn an enderman
Push a boat into him so he won't wander
Stack two blocks behind him and place a sign on top
Look at the enderman and run this command:
/effect give @e[type=enderman] invisibility
Walk around to position his eyes in front of the sign
→❌ Notice the sign renders in front of the eyes
Can confirm in 21w13a.