Just in case anyone ever searches and has this problem…
Well Specialized emailed back after a couple of weeks to say this is a known issue, and they’ve made a new wedge/bolt which doesn’t shake loose. Original was a touch lighter and made from titanium. They sent out a replacement for free, although I haven’t tried it yet as the Loctite fixed it in the end.
Also! They followed up with a second email a week later saying they’d searched the product manager’s notes and found the slipping posts were fixed by increasing torque beyond 6.3nm as stated in the manual - you can go up to 8nm, but not to exceed 8nm!
This is something I’d done myself, and found that 6.5nm sorted it for me, though it’s now nice to know my post isn’t about to explode as I’ve exceeded the manual’s torque.
Hope this helps someone in the future!