Hello,
There is likely one of two possible causes for what your describing: either a small tear in the camera flex cable or a grounding issue with new screen. With the screen & camera working while just connected with cables, the camera cable is not bent much, so a small tear may still make a connection. The camera cable is bent backwards with screen installed, so a small tear may separate. Use a magnifying glass to closely inspect the camera cable.
To test for a grounding issue with new screen, test the camera on the "original" cracked screen to see if it works when installed.