The bug
Entities render too dark after they have changed their height. This probably happens because their hitbox intersects a solid block for a split second (see MC-165134).
This affects mobs in singleplayer mode, as well as other players in multiplayer.
To reproduce (mobs)
Make mobs walk from a non-full block to a full block, e.g. by luring them with food
→ ❌ Some of the mobs will appear dark for a short moment
Note: This cannot always be reproduced, it is best to spawn multiple mobs at once.
To reproduce (players)
Be on a server with two players
Player 1 walks from a non-full block (such as a slab, an enchanting table, farmland, or a grass path block) onto a solid block
→ ❌ The player will appear dark for Player 2
Video
[media]Linked issues
Attachments
Comments

Included mobs (e.g. villager) as well since they are affected too