u/Boring-Day2692

So I don't know why, but I ended up buying a cheap 3DS off someone on Marketplace. For some reason the bottom screen is dark at first I thought the screen was broken, but I ended up playing with the Luma3DS screen filters and it fixed the issue in 3DS mode. But why is this happening? Is it an aftermarket panel? The screen clearly isn't broken. I can't seem to fix it for DS mode though I've tried TWPatch but it doesn't help the bottom screen at all.