The touchscreen is connected to the switch via a ZIF connecter on the cartridge reader board, and as it seems, it would be unresponsive on certain areas or register ghost touch even if you just simply disconnect and reconnect the touchscreen flex cable. I had that problem as well, when I was swapping the outer casing of the Switch with a transparent one. (Really cool, by the way) Anyway, seems that the cartridge reader board that you bought does have a defective ZIF connector, as you said that the touchscreen works fine when you put the old one back. It might be too late to say this, but maybe you can get a refund or something? If you can't, then I suggest you to check the ZIF connector on the cartridge reader board and clean it with isopropyl alcohol. You might also want to check the press connector that connects the cartridge board to the motherboard, as it transfers the touchscreen input to the motherboard. Or, you can simply use the switch with the touchscreen flex disconnected, since basically everything works with joystick input.