I used the model you’ve provided. They are not created through a script.
Here is a file with a Dogu15 rig and an S15 rig.
SkinnedChars.rbxm (56.3 KB)
You can open the animation editor with both of them as I included an animation that exaggerates the issue so you can see it easier.
One of the causes I saw was that it only occurs when Texture instances are applied to the surface, otherwise the seems will not be visible. I dont have much more to say on how it happens cause I dont really know.