This is what I (and some other witnesses) think has happened.
When we examined the item in question we were surprised to find that the rear peg of the vert. fin is not a rod, but atube.
I had not noticed it before, assuming that it also was a solid rod (like all the others, wings and horizontal stabilizers).
Effectively, when you build the model, you spend little time with the vert. fin : you just screw in the servo, link it to the rudder,and that's it....... You put the item aside until you fix it to the fuse before you go flying.
If I had noticed (and I could kick myself now for not doing so), I would have filled the tube up with a wooden dowel or a carbon rod, to make it solid.
Because, when you fix the vert. fin into the alu. retaining blocks and tighten the screw, the rod/tube is squeezed tight. I'm afraid that, in our case, being a tube, the item effectively gets a bit squashed and is weakened by it.
Being a really large fin, the tube is not strong enough to withstand the loads during flight and breaks. You can actually see the shape of the edge of the alu. retaining block in the breech-line of the carbon tube.
Since the vert. fin is now attached to the fuse by the forward carbon rod only (solid, but much smaller diameter), it folds under the pressure.
In the case of my maiden flight, I had a lot of down-trimming to do after take-off. I had reduced throttle, but, unnoticed by me, the airspeed seemed to be too much reduced for the model before taking the first turn. To my surprise it went into a mild spin (in my opinion, the model will stall rather easily, because the airspeed was not really that low). I think that this occurrence, normally rather harmless, already put too much strain on the tube in question.
I think the final result is obvious in the video.
Also, just to check my theory, I retro-fitted the vert. fin of the first airframe (which had survived unscathed). I pushed it laterally until the rear tube effectively broke, and it really didn't need too much pressure. Like its predecessor the fin then folds over, the only reinforcement being a horizontal rib about halfway.
So, to conclude : before you do some fancy flying with the M346, please make the rear carbon tube of the vertical stabilizer solid.