Clearly, the order of reassembly is the root issue! You always connect the MagSafe and/or Battery last!
They are shocking the logic board at which time the MOSFET’s get damaged depending on what was connected. In this case its the display, I’ve also seen this when a fake MagSafe charger is plugged in and then the DC-In board or I/O board is connected (live).
I think you’ll need to post a note in your returned boards on expressing this.
Independent of that you should invest in a few displays/shells so you can test the logic board fully before you ship the fixed board back. Or, just tell them you need the full system (which is what I would ask for).