OK, it’s not that the backlight doesn’t work. But I can’t decide if there’s a specific part/algorithm of what I intended that is flawed or not. I need to look into the details some more.
Your green layer, called “2”, has this fully opaque area at the bottom 1/4 of the frame. Also your layer opacity is set to 100%. This is why it doesn’t allow what you draw in that part of the frame to be visible. If you either turn that layer off or just lower the opacity (temporarily), you can see your drawings on top through the backlight.
This is an interesting special case. I’ll dive deeper into it soon.