To reproduce this bug, use zombie spawn eggs to create dozens of zombies. I expected a few of these zombies to be zombie villagers, but this was not the case.
Monster spawners appear to work fine; standing next to a zombie spawner (or cluster of zombie spawners) will eventually spawn a zombie villager.
Edit: It appears that zombies and zombie villagers are internally seperate mobs, so this my be intentional.
Yes, they are separate mobs. Zombie villagers have their own spawn eggs.
For natural zombie spawns, there is a section in the zombie.json in the vanilla behavior pack spawn_rules folder that sets 5% of zombie spawns to spawn zombie villagers instead. Dungeon spawners are either governed by this or hard-coded to the same %.