When zombie villagers, husks, drowned or zombified piglins are damaged, they spawn regular zombie reinforcements

The bug

When zombie villagers, husks, drowned or zombified piglins take damage, there is a chance (depending on difficulty setting it seems) that a zombie will spawn.

It is not clear what the expected behavior for these zombie types should be: Spawning zombies of the same type or not being able to spawn reinforcements at all.
However, for zombie villagers spawning zombie villager reinforcements might allow building a farm based on this, which might or might not be desired.

Note: For zombified piglins ("zombie pigmen" at the time) this was partially fixed in 20w14a by setting their base zombie.spawn_reinforcements value to 0. However, it appears that if a zombified piglin spawns as leader ("Leader zombie bonus"), then it is still possible for it to spawn zombies. The same also applies when setting a custom zombie.spawn_reinforcements value in the NBT or via commands.

How to reproduce

1. Be in the nether or the end (the overworld works too, but there will be natural zombie spawning as well)
2. Set the game difficulty to hard, and ensure the gamerule mobGriefing is set to true
3. Spawn a bunch of zombie villagers, husks, drowned or zombified piglins. Example:

   /summon minecraft:zombified_piglin ~ ~ ~ {Attributes:[{Base:1.0,Name:"zombie.spawn_reinforcements"}]}

4. Go into survival mode
5. Damage the mobs for a while
   → Zombie reinforcements are spawning

Expected behavior

Regular zombie reinforcements would not be spawned when entities that aren't zombies with the minecraft:zombie.spawn_reinforcements attribute are attacked. Instead, for example, if a zombified piglin with this attribute were to be attacked, it would summon zombified piglin reinforcements and not zombie reinforcements.

Code analysis

Code analysis by Marcono1234 can be found in this comment.