In my survival game (hard mode) I observed that a zombie spawner never stops making zombies even if the population of converted drowned rises well above any mob cap that may be in effect. I haven't noticed this happening until recently. I also haven't spent much time in my flooded dungeon drowned farm, so I don't know when this started happening.

Instead of harvesting any of the converted drowned, I decided instead to manage the items in the collection chest: smelting gold and chainmail armor into nuggets, keeping a few interesting enchanted pieces and discarding the rest. After 30-45 minutes I took the attached screenshots, showing (a) a view of the drowning room with what seems to be about 50 drowned crowded into it, most of them packed into the left end, and (b) the spawner dungeon room crowded with over 40 drowned that managed to climb on top of other drowned to get back into the room from the drowning room below (with a few zombies visible). It took probably 30-45 minutes to accumulate this many.

This farm is beyond the despawning range from my home base, so whenever I visit the farm, it's empty but starts producing zombies when I arrive.  At this point I'm using the farm only as an inefficient source of iron because I'm playing an island survival game ("Ocean Monument Ahead" seed from the seed picker) and there are no villages around to construct a real iron farm, and I've already mined much of the iron around my area.

Update: Still happening in 1.17.0.