When I change the tint color of the mesh material, it will create an instance material for that mesh correctly. However, I found that in some cases, spine will reset it to the original shared material so my tint setting is reset. For example, if the animation has attachment visible keying, the reset will happen at that time.
Demo: I was changing the tint color on update.
Another case is by having mask (clipping), I cannot change the tint color at all because the material will reset instantly.