I've got a 1989 BX GTI, I've got a similar problem except my courtesy lights work fine off switches and the rear doors but not the front doors.
To confuse things further, the little display next to the ash tray that tells you which doors are open still works perfectly!
That display works OK on mine too!
I would have thought that your door switches for the front doors are dirty or corroded and failing to earth. Quite simple to replace or clean them. I assume your delay is working if you open and close a back door? By which I mean the int lights stay on for a period after you close the door.
I am wondering if the replacement timer that I fitted is duff, or if there is some other gizmo somewhere in the circuit which may be defective. Local Citroen garage failed miserably when asked to correct this fault of course.
You don't happen to have a manual, I suppose? Love to see a wiring diagram!