The bug
When spawning a mob on a block with a fence on it the mob will spawn inside the fence.
How to reproduce
Put a fence on a block
Use a spawn egg on the block that has the fence on it
→ ❌ Notice mob is inside of fence
Linked issues
is duplicated by 7
MC-271485
Zombies can Spawn inside fences via Spawn Egg
Resolved
MC-137503
Mob can spawn in fences and walls.
Resolved
MC-270616
Mobs can be spawned inside of fence.
Resolved
MC-213403
Mobs can be spawned inside of fences and walls using spawn eggs.
Resolved
MC-160396
Entities run through fences when they are spawned on the same block
Resolved
2 more links
Can confirm in 21w03a.