Looking more closely at the material graph I think it’s just an error there. You’re taking a texture file that has a thick white border (value of “1”) and using “add” to put it on top of your main texture map. So those border pixels will always be greater than 1. If you want to use it as a mask, you can put it into a “Lerp” node to blend / stack textures on top of one another. Or you can use “Multiply” to turn all the inside pixels black and leave the outside pixels the same value.
So get rid of that Add.