The Bug

Due to the fact that the game saves a multi_noise biome layout in level.dat, when the game is upgraded from 1.17 to 1.18, the stored biome layout becomes invalid, and the game reverts to a multi_noise configuration with 0 biomes. This prevents a world from being loaded, even when the included datapack has been appropriately updated to 1.18's data format. This means that any world which had a datapack containing a multi_noise biome layout in 1.17 cannot be upgraded to 1.18. (A fix for this bug does not require the multi_noise configuration saved with the world to be upgraded. It is sufficient if the upgraded world is reverted to the default 1.18 multi noise configuration.)

A log containing the output of the game after triggering this bug has been attached to the bug report.

Intended Behavior

The world should be able to be upgraded normally if existing 1.17 datapacks are replaced with datapacks following the 1.18 data format.

To Reproduce

1. Create a new world and install the attached datapack terralith-v1-6-4.zip during world creation.
2. After creating the world, exit the game and load 21w44a.
3. Delete terralith-v1-6-4.zip from the datapacks folder of the world.
4. Install the attached datapack terralith-upgrade-path.zip into the world's datapacks folder.
5. Attempt to load the testing world and note that the world is impossible to load.