The most likely problem here is that there's some damage to wiring between the door and the body control module (BCM) or a fault with the BCM itself. The front BCM is the most likely culprit.

It's a rare but known problem that can occur on all derivates on 3 and 4-Series, usually where wiring has chafed in the door shut area and made a bad connection, but sometimes where the BCM has sustained water damage. Being a known issue on 2013-onward 3 and 4-Series models, and with your car now being out-of-warranty, you should be able to get it investigated much more cheaply by an independent BMW specialist.