The issue is as presented in the title. Steps to replicate: Go and generate a new chunk, then, place a powered belt right up against the chunk border, then drop a few blue slimy ferns on the belt, and save and quit. Upon re-entering the world, the file can no longer correctly load, and results in the user being stuck in a entirely unloaded world. The process seems entirely broken, as trying to exit the world results in the process getting stuck, and needing to be terminated via task manager.

I have tested multiple variations of this, and it seems to be necessary for it to be blue slimy ferns for some reason. additionally, it might be that instead of non-spawn chunks it requires a certain side of the chunk border, but I have not tested.

This is a screenshot taken right before saving and exiting the world. Upon reload the world was no longer viable.

The same setup, in a spawn chunk, and maybe on the wrong side of the chunk did not result in a corruption. Note, I also tested this with slimy fern, and that likewise did not work. Its not shown here, but upon moving to a non-spawn chunk, and onto a different face, I tested with both cobble and slimy fern, and only the fern resulted in corruption.

I have lightly modified the modpack, adding CobbleForDays, and a single recipe to kubejs to allow the smelting of gravel into andesite.

Log file for that last corruption, upon loading, and quiting, listed here.
latest.log