Make sure all of your UVs are in the 0-1 space, if you've got the set up like UDIMs, ie: 0-1, 1-2. 2-3, 3-4, this won't work with the texture set feature. UVs outside of 0-1 will be ignored.
I had some shells that were falling out of the 0-1 space and fixed those. I'm still seeing the same error, however. I'm thinking of making the part causing the issue a seperate model entirley. Here's currently what my one of the UV sets looks like: