So there appears to be a little bit about this issue around on the internets, problems going back a few versions of iOS at least. Generally speaking, you are pointed toward the settings to check that auto lock is actually turned on.
In my case, I’ve had a few times now when I have put the phone down (or onto the charging dock) and it’s simply stayed awake, screen on full brightness… and nothing. I’ve had it happen both on and off power too, so it’s not limited to when it knows it’s on charge. Changing the timeout to another setting then back to 1 minute (my setting of choice) didn’t kick it back into life.
The problem is solved for the moment, a re-boot of the phone saw it come good (again) but this has happened twice now. Is anyone else seeing this behaviour?