The following steps were updated on 8/15/24. The original steps no longer made the issue evident after the increase in the cave monster cap in 1.17.40, simply because the cap-related math in those steps was no longer accurate. There has never been any change to the actual behavior reported here.
Steps to reproduce (updated 8/15/24)
At Y = 300 (to be out of range of any other spawnable spots) create a spawn platform about 20x20 with 2-block high walls around its side, and no ceiling.
Get yourself a nametag.
Set time to night and turn off daylight cycle.
Move far enough away from the platform for mobs to spawn on it.
As mobs spawn, kill all non-zombie mobs (including zombie villagers) and nametag all zombies.
Once you have 8 zombies, fill the platform inside of the walls with 2-block high water.
Make another identical platform a few blocks away, on the same Y-level.
Move far enough away from the platform for mobs to spawn on it. After all of the zombies on the first platform convert to drowned, wait about another minute.
Expected results
No monsters spawn on the second platform. The zombies that converted to drowned continue to occupy the surface monster population cap.
Observed resulys
Monsters spawn on the second platform after the zombies convert to drowned. The surface NBT bit does not carry over during mob conversion, so the converted drowned count toward the cave cap.
Note on impact:
Due to this bug, zombies can continually spawn and drown, building up massive numbers of drowned mobs in surface pools. This consequence was reported in MCPE-34032, which was resolved as "Fixed" after the despawn rule changes in 1.16 made the issue with drowned less severe. However, problem with mob build-up still occurs as described in MCPE-125111.
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Auldrick
Confirmed in 1.14.20 Hotfix on Windows 10.
I tested this by summoning a zombie on the surface, pushing it into a river, and waiting for it to become a drowned. I then used a third party tool to look at the Surface bit in its NBT data and found it set to 0, indicating that the drowned was being counted against the cave density cap. All drowned are supposed to be surface mobs.
GoldenHelmet
@Auldrick, I'm not sure how you summoned the zombie, but I intentionally used naturally spawned surface zombies in my tests and reproduction steps because I have found that spawn eggs always produce cave spawns, and I wanted to be sure that the converted drowned were not just carrying over the cave tag from the zombies that drowned. So my setup proves that (1) converted drowned do not carry over the NBT surface bit from the zombie, and (2) converted drowned do not get assigned a surface bit value based on the block they spawn on having sky access, and (3) converted drowned do not get assigned a surface bit value based on the rule that drowned are only surface mobs.
GoldenHelmet
This bug probably affects husks converting to zombies as well, but my test environment does not naturally spawn husks, and I am not command-savvy enough to quickly make another test setup in a desert. The fix for his bug should be applied to husk-zombie conversion, and probably to pig-pigman and villager-witch conversion as well (because why should a thunderstorm fill up the cave density cap?). Thunderstorms I can test more easily and will update when I do. This bug does NOT affect villager-to-zombie villager conversion because of another bug, MCPE-62030.
Update: converted zombie pigmen and witches count against the cave monster density cap, too.
Udpate #2: medium slimes that spawn when you kill a large surface-spawned slime also count as cave mobs. (I observed this in my witch hut testing area, which is similar to the test setup described for drowned above but built around a witch hut in a swamp biome. I ran a /kill @e command when only 1 slime was on the surface platform (Y=69), which caused 4 medium slimes to spawn on the surface platform. I then waited for further spawns. 8 new monsters spawned on the surface platform where the medium slimes were, and only 4 monsters spawned on the cave platform below.) If this is fixed so that the medium slimes spawn with a surface tag, you should also make sure that the small slimes that spawn when you kill a medium surface slime will spawn with a surface tag as well.
[Mojang] Mega_Spud (Jay)
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However, this issue has been temporarily closed as Awaiting Response
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GoldenHelmet
Still can reproduce.
migrated
I'm seeing this in 1.18.0. Many, many drowned have accumulated in a couple small ponds just outside of a village. I've been building and spending a lot of time AFK at a farm about 4 chunks from these ponds (this is on sim 6). I'm estimating about 30 drowned are crowded into the 2 ponds. The drowned are not able to walk out of the ponds because the water is at least 2 blocks deep everywhere.
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It looks like every night any zombies that spawn in the area are attracted to the village, then in the morning they seek water to avoid burning up and some of them make it to the ponds. The zombies drop their armor when they convert to drowned, and then some are able to pick up the armor and obtain persistence. There doesn't seem to be any game mechanic that stops them from building up more and more given that they all start as surface spawns but then become cave spawns once they convert.
migrated
I added a couple of screenshots. You can see that they all have equipment and/or an item (string, raw chicken) or weapon. The items and equipment were all found without player intervention.