Major chunk flickering in custom dimension

When exploring a custom dimension, chunks started flickering, and became "hollow". The blocks where all still there and there was some lag. The debug log gave no errors, and even previously generated chunks flickered.

How to reproduce
1. Download the provided data pack
2. Create a new world with the data pack
3. Type /execute in fsb:checkerboard run tp ~ ~ ~
4. Fly around for a bit, then notice that chunks start to flicker and stutter