I've got the switch in the centre position.
Yes, I've got a starter button - I think that's the issue. Automatic activation is triggered when "The ignition key is removed (vehicle without starter button)" according to the manual.
I've seen other posts in other car forums and it appears this is the norm now. VW and Volvo owners have reported that on newer model versions the interior light doesn't come on when the engine is switched off. Older model versions it did.
It's not a big issue with me - I just got used to my last car, (a Fiesta), that had this feature as well as better ambient lighting.