I can't exactly wrap my head around why this occurs, but I will try to at least try to pinpoint what might be causing certain issues.
So it might be a crash with BlockStates or Models most likely, but I still don't know. Here is how to and what it does.

How to reproduce:
Get a pack I made (BreakMe)
Then try loading it.

A video is attached:

Here is also a crash log:
crash.txt

Please note the name of the pack

I do think that resource packs that put textures into invalid states should be handled much more elegantly than just crashing so that will likely be done