The Bug

The glowing effect causes some types of mobs to have black dotted lines floating next to them. For some types of mobs the lines are obvious. For other types of mobs the lines are only a few pixels. The spacing between the dots in these lines changes depending on the angle you view the mob from. These black dotted lines are visible through blocks, just like the normal outline for the glowing effect. If the glowing mob is on fire, an additional black dotted line appears next to the fire texture.

To Reproduce

/summon villager ~ ~ ~ {NoAI:1,Glowing:1}

There are two perpendicular black dotted lines floating next to the villager’s head.

Affected Mobs (There might be more.)

Bee
Cod
Creeper
Donkey
Drowned
Enderman
Horse
Husk
Parrot
Phantom
Salmon
Skeleton
Slime
Stray
Trader Llama
Villager
Vindicator
Wandering Trading
Wither Skeleton
Zombie
Zombie Horse
Zombie Villager