Ahh, I see what your issue is.
Clamping will only display where there is any alpha behind it (all layers below). So when you turn the BG layer on, there is alpha behind all your overhang, and therefore it will display.
In my “magic” BG layer, I have actually used a separate GP object.
Ouch, I hope you didn’t travel with that tooth rot guy for too long. I’m going to brush my teeth again right now.
