Thanks for the replies. To confirm in the menu it is set to be on all doors (no single side option).
As you mention @dcw I did suspect it is probably corrosion, I'm guessing the dealer service bulletin (in linked thread) doesn't mean there's a FOC fix?
It just seems strange it's only on the one side, which got me thinking is it some kind of switch or something that can be checked on the car? I did think a fuse initially, but there's just a single one for the whole keyless system.
Wiring diagram is on page 100, however there seems no link between the two?