I have the same problem here.
I had an official 7'' touchscreen display working with a RPi 2B and couldn't get it working with the RPi 3B+.
At first, I thought the position of the connectors was switched due to some board design reasons, but then I've searched the official documentation for the Raspberry Pi 3 B+ and confirm that my board's DSI connector for the display, as well as the camera connector, are mounted 180º from the documented design.
I'm going to report this to the distributor who sold me the board, in order to get a replacement.
Am I missing something here, or is this a fault on the assembly?
Thanks in advance.