I have a tailgate so not 100% sure if theres any difference between that and barn door but my works as expected van will auto relock after 30 secs if no door is opened
The central locking can be setup differently so the rear/SLD is separate from the front doors, not sure if this is the norm on panel vans but certainly not on a kombi
It is possible your door contact switch is the problem and the vehicle is not registering that the door has been opened, do the interior lights come on when you open the back door ?