The Bug
Normal zombies can spawn reinforcements in biomes other than those which zombies cannot naturally spawn (such as the End). This behaviour is marked by MC-227735 as Works As Intended. Drowned can only spawn reinforcements in the biomes that they can spawn naturally. MC-275544 supposedly fixes drowned reinforcement spawning, as it was previously broken, however this fix only allows drowned to spawn in river, ocean, and dripstone cave biomes.
How to Reproduce
Create a superflat world with three layers of water added on top
Set the time to night
Spawn a drowned that can spawn reinforcements using this command:
/summon minecraft:drowned ~ ~ ~ {attributes:[{id:"minecraft:spawn_reinforcements",base:1.0}]}
4. Attack the drowned with snowballs
Expected Result: Drowned reinforcements should spawn
Actual Result: No drowned reinforcements spawn
Other Notes
I have not tested this with zombie villagers, husks, or zombified piglins. They may also be affected in a similar manner.
Comments 2
This seems to be fixed in 1.21.2-pre4. Not sure why as nothing in the changelog mentioned anything that fixes this, and this bug report (as of writing this comment) is still unconfirmed and unresolved.
Edit: this actually does work in 1.21.2-pre3. Not sure how I missed it before since I thought I tested it pretty thoroughly.
Can confirm