560SEL passenger door lock problem

[Thomaspin]

I have a strange passenger front door lock problem on my 1990 560SEL.

Central locking works fine when I'm the only one in the car, meaning only the driver's door is opened/closed during the trip.

However, when I have a front seat passenger accompanying me, after leaving the car the passenger front door will not lock on the first central locking attempt. It does lock after a second attempt. The behaviour is consistent.

Is this as simple as a failing vacuum element, or is there something deeper going on here?