It seems like I read about this same problem on a thread here a long time ago, but I can't seem to find it with a search. I backlit and biverted with HHL supplies, followed the YouTube instructions they link to from This Does Not Compute. As I set it, the polarization should have it normal, not inverted.
Starting with the contrast pot rolled all the way toward the top of the dmg, the faintest shadow of an image is visible. As I roll the pot down, an inverted image appears though not a really nice looking image. As I continue to roll it down, the image almost immediately stops looking inverted and gets to looking almost acceptable but nothing near as good as all of the other modded gameboy a I've ever seen. It basically disappears rolled down anywhere past that.
I feel so sure I've heard of others encountering this same issue. Does this sound like a bivert module issue? Cap replacement necessary? Damaged led ribbon? It' so close to great, I really am puzzled by this. I've got a video of the behavior, I'll try to figure out how to post it.