I would think, if the display ribbon cable was in wrong, that you wouldn’t get any backlight?
I don’t have one but it’s my understanding that the cable goes in under the black locking bar, contact side down facing the driver board.
Do you have the other board with the buttons connected to the driver board? I honestly don’t know if its needed, but if you didn’t hook it up, I would.
Somebody else reported a Plasma issue like yours. Might be worth reporting it on GitHub.
Failed to start Plasma LED sequencer daemon - Picade - Pimoroni Buccaneers