This will be an unsolved mystery 🙂
I was already thinking about going 1x and fairly sure about it so it’s done now. In the course of doing that I had to strip everything off and I can’t see a specific problem, everything looks fine and I can’t see any actual problem with the BB which is 100% the correct one as I contacted Ghost to verify I’d not got the wrong one.
Although it looked OK part-by-part, the derailleur position just wasn’t right at all, there’s no apparent damage though. Anyway, it’s history now 🙂