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

1. Create a superflat world with three layers of water added on top
2. Set the time to night
3. 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.