The game can no longer save chunks in custom dimensions

The Bug

When saving a world, any chunks (and points of interest) in a custom dimension can no longer be saved to disk.

This issue first appears in 25w43a.

Edit: I have figured out that this issue only affects custom dimensions; whether the dimension type is custom or not does not seem to matter.

To Reproduce

I’ve provided a data pack with a custom dimension named example:test.

1. Enter the “Create New World” menu and enable “Allow Commands.”

2. Install the attached data pack.

3. Set “World Type” to generator.example.test.

4. Create the world.

5. Teleport yourself to the custom dimension:

   /execute in example:test run tp ~ ~ ~

Observed Results

An error message box pops up in the upper right corner, saying “Failed to save chunk at [<x>, <z>].”

Upon saving and quitting to title, the game gets stuck for a long time before returning to the main menu.

Expected Results

There should be no error messages and the world would be correctly saved.