I thought the weird ones all fell my way.
Is it a S1.5 with Smartshield, not Smartlock?
If all doors
unlock, and there was no change with a substituted lock, then you have integrity of wiring. The same pair in each door either lock or unlock depending on which is assigned each of the ground and positive.
The final test of this would be to "direct fire" the driver's door via the kick panel junction after tricking the lock shut, with door open. I touched on this in another thread but you would need to identify the two pins for the driver's door - they'd likely not be the two I used for the passenger unit. Hotwired like this both lock and unlock should be 100% (but on just the driver's door).
I'm going to guess that despite all doors operating on one signal, it has two controller relays - one for the driver's door and one for the rest. Following from this surmise, the driver's door relay may be faulty.