Affects Version/s: 1.17 Release Candidate 2, 1.17, 1.17.1 Pre-release 1, 1.17.1 Pre-release 2, 1.17.1 Pre-release 3, 1.17.1 Release Candidate 1, 1.17.1 Release Candidate 2, 1.17.1
Fix Version/s: None
Zombie reinforcements can spawn on the following biomes: Mushroom Fields, Mushroom Field Shore, The Void, and all Nether and End biomes, even though zombies naturally don't spawn in those biomes.
1. Create a single biome world of one of the previously mentioned biomes, on creative mode.
2. Set the difficulty to Hard.
3. Set command blocks to constantly give a high level of Resistance to all entities and kill any mob that is not either a zombie or the player (you cannot execute the command /gamerule doMobSpawning false, as that disables reinforcements). This is not necessary, but it assures that other mobs won't interfere or attempt to kill the player.
4. Set the time to night.
5. Create a barrier enclosure, and spawn some zombies.
6. Switch the gamemode to survival, and hit the zombies.
7. Zombies will spawn outside the enclosure.
Could be intended behavior.