Resolution: Cannot Reproduce
1.15.2, 20w06a, 20w07a, 20w08a, 20w09a, 20w10a, 20w11a, 20w17a, 20w19a, 20w20b, 20w21a, 20w22a, 1.16 Pre-release 2, 1.16 Pre-release 7, 1.16.1, 1.16.2 Pre-release 1, 1.16.2 Release Candidate 2, 1.16.2, 1.16.3, 1.16.4 Pre-release 1
[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
Affects Zombified Piglins, as they can summon normal zombies too.