I created a 1.20.1 resource package using the custom model data feature.
All paths of the model are normal and can run normally in the blockbench.
You can open the JSON file of the model and see that the texture and model are completely fine.

However, in the 1.20.1 environment (all modules have been removed, including openfine). There is no problem with the model,
But the texture completely turned into purple black squares (damaged by collision).

Interestingly, at 1.16.5, everything in this material bag was normal.
I don't know what problem caused it.

Interestingly, there is no problem with the blocks, it's just that the  items has this issue.

What's even more interesting is that the 2D model is great, but the texture of the 3D model crashes in 1.20. x.

This is the material package: test.rar

Please enter the following code to obtain the Iron Sword with a custom model file:
/Give @ s minecraft: iron_sword {CustomiModelData: 8}