[RobbiZ4]

Well, that sounds curious.The harness is one single piece in a Y-form.

You can't separate it easily as you have to remove tapes and cut or un-pin and re-pin wires. But it might be an option to gain some time as the remounting takes about 3-4 hours on each side of the car.

In general, this doesn'n make any sense. And it could be the source of your current issue!
But it might have been an option for the dealer to make some money without having to work. If so, kick him into the ass!

I assume it's difficult for you to check if both sides were replaced. When was it replaced exactly?
But I remember a part of the harness that looks different between the old and the new one, so you should have a look at it as well.

If it looks different, you have to go back to the dealers...