Help with uv_1

Hello there! So i've been encountering this issue with making cc for horses: In game my mesh behaves as if it doesn't has a uv_1 even tho it clearly has one. I've tried so many ways to fix this but i just can't seem to find one that really works so I thought i would ask here. Other creators also seem to have this problem so might it be a problem that lies with s4s? 

Here's the Link to the Files: drive.google.com/drive/folders/1hBeWvb-tzwnN_kNUkEiymRcB60Gv6gQD?usp=sharing

This also happens with anything else created for Horses not just tack.