The bug

MC-84121 was fixed in 21w10a. However, if the mob is invisible MC-84121 still occurs. Invisible glowing sheep do not have their wool outlined. Invisible glowing mooshrooms do not have their mushrooms outlined. Invisible glowing slimes only have their inner part outlined. Invisible glowing snow golems do not have their pumpkin outlined.

To reproduce

1. Build a wall:

   /fill ~ ~ ~ ~ ~5 ~30 minecraft:black_concrete

2. /summon minecraft:sheep ~ ~ ~ {NoAI:1b,Glowing:1b}

3. /summon minecraft:mooshroom ~ ~ ~ {NoAI:1b,Glowing:1b}

4. /summon minecraft:slime ~ ~ ~ {NoAI:1b,Glowing:1b,Size:3}

5. /summon minecraft:snow_golem ~ ~ ~ {NoAI:1b,Glowing:1b}

6. /effect give @e[type=!minecraft:player,distance=..30] minecraft:invisibility 9999

7. Repeat steps 2, 3, 4, and 5.
8. Go to the other side of the wall. Notice that the invisible mobs have different outlines than the visible mobs do.