Minecraft 1.20.1
Forge - 47.2.6 - 47.2.8 (Not tested on prior versions
Instance - Server environment

Using the mods

YUNG's Better End Island (V 2.0.4
&
Savage Ender Dragon (DragonFight) - SomeAddon (V 4.1)

Causes endermen to be invsibile. It registers their UUID and records show that they spawn but they never do. Using eggs also doesn't work. It makes the endermen sound and registers in the logs (If you have a mod that shows them) but they don't appear in game