Can you provide the FBX file?
Sure thing, there it goes as exported from blender. Thanks.
Could be because your uvs are outside of their 0 to 1 space.
Oh, I'd really enjoy a 5-year-old appropriate explanation on this. I'm really new to texturing.
EDIT:
So I just imported the very same .fbx to blender just to check, and yep, it shows me the stretched UV map, same as the one in Substance Painter.
