After the conversation I realised that I hadn’t tried the bag on another rucksack (with wider straps) to test whether is was the buckles, the straps or combination of both. At home I fixed the bottom right buckle on a Berghaus 64zero that has straps that are approx 3mm wider than the Exos.
The buckle with some load and the strap clearly not contorting.
The same again but on the strap on the Exos.
Clearly the positioning of the slots and the width (and possibly thinness of the material) of the straps on the Exos mean that when loaded, that is tightened into place, the straps contort and move into in a position that allows escape, which is in fact what they do. A design fault then which, I believe, can be remedied by using buckles with the slots at the other end to where the strap tightens against.
I’ve written to Osprey to point this out and I hope that they take this on board and sort for future production of the accessory.
Unfortunately for me the buckles aren’t reversible, so I cant pick them apart and re-stitched the other way around. But I have found a simple solution.
I cut two one inch lengths of wider black strapping and inserted these between the Exos strap and the buckle, this effectively blocks the slots and helps prevent the Exos straps from twisting. It don’t look that tidy but at least I rescued the product from the not to be used ever again box in the attic.