-
It's a very near estimate and if anything like @MisterMikkel said tyre sag will probably take it to that 31 mm even if the tyre isn't as high as it is wide. OTP bikes' bottom bracket are high so you don't get pedal strike while cornering or while you're out in the woods with squishy capable tyres. Moreover lowering the BB by altering the geometry would slack the frame and alter the ride even more for you'll have to lengthen chainstays & DT. You may personally like how your straggler rides with smaller wheels (I call placebo but I'd be interested to try). All this makes me think going from 622-40 to 584-47 definitely makes less sense than putting 584-38 or 42 mm on a bike that wouldn't take anything larger than 622-28s, for you more or less keep the same ride. ✌️
Heldring-
Pedal strike was never an issue with the Straggler (while I often scrap my flats on my daily). You could argue it's all in my head, or a confidence thing, but ultimately I corner better on lower BB bikes (and yes I have tried old school geo CX race bikes on the same sort of terrain too). I'm not saying the Straggler on 650B was perfect, and I found its front end a bit vague, but it was super stable and it could corner faster than everything else I've ridden before or since.
Lolo
IRL it would be less than 31mm (it's rim dependent but the tyre nominal width does not equal tyre height).
Plus, as Ed said, a lot of OTP bikes have a way too high BB to start with. I much preferred the handling on my Straggler with 650B wheels, although it was designed for the same sort of volume on 700c.