Hello there!
I'm actually trying to modify .mca files in the region folder of my custom dimension.
But suddenly, A lot of error messages started to appear in the game output, and the chunks didn't modify.

Steps to follow:
1: Create a custom dimension with data packs.
2: Enter in the custom dimension and charge some chunks.
3: Leave the game and modify the chunks on that dimension, (from other world for example).
4: Enter in the game and load the custom dimension. You will see a lot of error messages in the output and the chunks won't load.