Since commit 440547d, enderman have not been able to spawn in the end dimension.
I have tested this on newer commits aswell, but that doesn't change anything.

The end world was also tested when completely regenerated, without any other mods installed - and reverting to an older commit afterwards seems to fix the issue aswell.

I've been able to reproduce this issue.

This issue seems to because caused by the new get RawBrightness method that was added/moved to this class in the commit. This method changed where it got the sky light value from, that being getSkyLightValue() as apposed to where it used to get it: skyReader.getLightValue().
This change causes this issue because getting the skylight value from the former method has no check for whether the dimension actually has a skylight, where the latter method does. This causes The End to have a sky light of 15 and subsequently prevent endermen from spawning, as they can only spawn in a light level below 12 or whatever.

I reverted this change in my fork and it seems to fix it.

e9ff6b7