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)

1. 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)

1. Be on a server with two players
2. 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

Minecraft 19w45b 2019-11-10 16-59-31_Trim.mp4