The bug

Starting with 20w18a some mobs do not count to the mob cap anymore if they are a passenger (e.g. riding a minecart).

The following mobs are not affected:

• Mobs which can be part of a raid (e.g. witch)
• Fish

Reproduction steps

1. Open the debug screen (F3)
   The following steps refer to the "M:" ("Monster") mob category shown in the line starting with "SC:" on the left below "Local difficulty:" and above "Sounds:".
2. Spawn a husk
   The husk is counted for the "Monster" category
3. Spawn a husk riding a minecart

   /summon minecart ~ ~ ~ {Passengers:[{id:husk}]}
   

   The husk is not counted

4. Destroy the minecart
   Now the husk is counted

Code analysis

20w19a, Mojang names

In 20w18a net.minecraft.world.entity.Mob.requiresCustomPersistence() was changed to return whether the mob is a passenger instead of always returning false. This might have been done to fix MC-178013, since this method is also used to determine whether a mob can despawn, but has the probably unintended side-effect described in this bug report.

Additionally it is not consistent whether passenger mobs count to the mob cap, because the classes overriding requiresCustomPersistence() do not consider the parent result.