[SOLVED] Is it the weights, vertices, or uv_1?

Hi. The garment is not uv unwrapped in uv_1. Separate it from the body and transfer uv_1 from the body.

Also, the uvs of the garment in uv_0 are stretched. They were generated in the square uv space but CAS textures are rectangular, so when you apply rectangular textures to them, they stretch. You need to bring them back to the original size with S Y 0.5 before transforming.
And you'd want to delete all areas of the body covered by clothing because they serve no purpose