Summary:
When upgrading zombie villagers from before 1.9 to 24w18a, there seems to be a chance that the game crashes. It seems that the more zombie villagers there are, the higher the chance for the game to crash. If there are only a few zombie villagers, it usually doesn't seem to crash. This is probably because it randomises the profession when upgrading, since zombie villagers before 1.9 have no profession. This doesn't occur in 1.20.6. But in 1.20.6, when loading the world, the messages “Failed to load chunk at [X, X]” appear.

Steps to reproduce:
You can skip steps 1 and 2, use this world: 2024-05-04_20-53-44_New World-----.zip
There are many zombie villagers in the world so that the chance of crashing is almost 100%, but this can happen even from ≈30 zombie villagers.

1. Create a world in 1.8.9.
2. Spawn a lot of zombie villagers using the following command:

   /execute @e ~ ~ ~ summon Zombie ~ ~ ~ {IsVillager:1b}

3. Upgrade the world to the latest snapshot.

Observed result:
The game crashed.

Expected result:
The game should not crash.