Expected Results: When zombies convert to drowned, the drowned would count toward the same density cap that the zombie counted toward, OR when zombies convert to drowned while having sky access, the drowned would count toward the surface monster density cap. Either behavior would be consistent with naturally-spawned drowned only spawning as surface mobs. Either would also prevent the accumulation of drowned in pools and rivers by stopping further zombie spawns on the surface nearby. In other words, if 8 zombies drown in a surface pool, then no more surface monsters should be able to spawn in the density check area.

Observed Results: when naturally-spawned surface zombies convert to drowned with sky access, the drowned count toward the cave monster density cap. This allows more zombies to repeatedly spawn nearby and drown, building up the massive numbers of drowned reported by MCPE-34032. So, effectively, instead of a limit of 8 monsters on the surface within the density check area, you have no limit to the number of coexisting drowned the game can generate over time.

The accumulation of cave-tagged drowned on the surface also limits cave monster spawns below the surface, disrupting the balance that the separate monster density caps are designed to ensure. Further, it prevents deep slime spawns in slime chunks and witch respawns in witch huts, both of which are issues that have been repeatedly reported as bugs by the community (cave slimes: MCPE-49303, MCPE-47841, MCPE-61520 just mention a few; witch huts: MCPE-60552 contains links to earlier reports).

Steps to Reproduce:

1. Create a spawn testing world with 2 1-chunk platforms in the middle of spawn-proof 9x9 chunk area. Set up one of these platforms as surface and the other as cave.
2. On the surface platform construct a roofless chamber with 2-block highs walls to contain water, but do not place water yet.
3. Set time to night.
4. Wait for zombies to spawn within the surface chamber.
5. Fill the 2-block high surface chamber with water and wait for the zombies to convert to drowned.
6. Seal off the drowned chamber so that no more zombies can wander into it.
7. Kill all other mobs on both platforms, except the drowned.
8. Set time to night again and wait for spawns.
9. You will observe 8 monsters spawn on the surface platform, and 8 - # of drowned spawn on the cave platform. This proves that drowned spawned on the surface via the conversion of surface zombies nevertheless limit cave monster spawns and do not limit further surface monster spawns.

This issue may relate to MC-88967.