Morning Owen
Welcome to the Forum
Not a common problem but I have seen it twice once on a 5 series and once on my X5 in 16 years. Both cases it was a failed door actuator getting the door open is a Faff as you have to take off the door card to "A" get the door open and then "B" get to the actuator.
I undid the passenger seat and tipped it back to give better access and remove the door card. Then with a long screw driver manipulate the door lock mechanism and get it open. I replaced both actuators with second user parts at a reasonable cost. Tip : remember to plug the window switch back in or the Central locking wont work plus test with the door open.
I had scanned both cars first and both showed an Actuator fault
Dave