The bug

When a villager that has been traded with is zombified, it will not be able to despawn (see MC-159300). However, it still counts towards the mob cap.

To reproduce

1. Summon a villager and give it a workstation
2. Trade with the villager
3. Open F3 screen, and observe the mob cap line (starting with "SC") from now on
4. Summon a zombie to zombify the villager
5. The "M" value ("monsters") increases by 1 when you spawn the zombie, indicating that the zombie counts towards the mob cap
6. When the villager is zombified, the "M" value increases again
   → This means that the mob cap increased again
7. /tp ~ ~200 ~
8. Observe that the "M" value now decreased by one since the zombie despawned
9. /tp ~ ~-200 ~
   → The zombie despawned, while the zombie villager did not, but the zombie villager still counts towards the mob cap

Credit

Found by Rays in this video: https://youtu.be/uJVkdTpdI2o?t=559