Well at least it looks like they don’t all do that, doesn’t seem like anyone else has suffered the same misfortune.
Having re-read mine and the op post, as instructed, I now exercise my prerogative and change my mind..... I know you’re not allowed to do that on the internet, but I do that a lot, it’s quite liberating.

V8 Dave has my sympathies and since he didn’t want a free repair, the factory has missed an easy resolution. Some helpful insights as to the many reasons why the crack could appear, would have been simple to provide.
I doubt anybody could offer the actual explanation, without dismantling the car, it’s an
“ how long is a piece of string?” type question.
After eight years, I’d be surprised if it’s just a thin area on the panel, are you sure there isn’t some extra twisting of the chsssis or the wooden frame going on? Is something else broken or detached?
My car has seen some very bumpy and uneven roads, it creaks, shakes and squeaks in protest, but so far, no cracks.