The Bug

Entities appear dark after around 2050 blocks in the y level, including the player and particle effects such as potion swirls.

How to Reproduce

Teleport yourself up 2050 blocks using this command (make sure you are in flight mode in Creative before running this command):

/tp @s ~ 2050 ~

Note how your arm is dark. Going into third-person mode reveals that your player model is dark as well. If you summon entities, they will appear dark as well.

Going below around y=2045 returns the player model to normal.