Zombie reinforcements can spawn on top of Nether roof

[HARD MODE ONLY]

Summon zombie that is able to summon reinforcements of other zombies

Put it under nether roof (y=127-125 should work)

Go into survival mode and attack the zombie

The zombie can summon another zombie on top of the nether roof, might take a few tries

if zombie spawns holding an item (shovel, sword) this zombie will never despawn and will stay on the nether roof forever. 

The issue was already reported but marked as Invalid (see MC-159230)

Affects Zombified Piglins, as they can summon normal zombies too.

Relates to MC-169891 and MC-153787