Why are the flex cables shorting out?

Cloud of smoke? Definitely no good. Sounds like here are a lot of variables here since you swapped out a lot of components (honestly, that’s nigh on a whole new phone at this point) but here’s what I would check.

1. Is this the same front camera assembly from the original display that you transplanted? Does the front camera even work despite the other issues? What about the ambient light sensor/proximity sensor? If no, make sure nothing was damaged in the process. Check the cable closely and the cable connector for signs of tearing or damage. If the camera came as a part of the display assembly, my money is on that part is bad. See if you can transplant the one from the original display.
2. Do you know the new logic board is good? I would check the connector on the board and components around it. See if anything is getting hotter than it should.