I am not as familiar with these sort of problems on iPad, but it could just be that there is a conflict with pairing of the new camera assembly. I think Apple fixed this on the iPhone 12 with a software update, after backlash when unpaired cameras didn't work properly at launch. I assume you installed a new display as well?
It could also be this particular camera is faulty, have you tried more than one? I also generally have pretty good luck from mobilesentrix, but this just happens sometimes.
Otherwise, it's possible there may be damage to the camera connectors. Some pictures of this area of the board might help.