I don't understand your video Speed 21 back stall. Why would be using a 21m in no wind fully depowered? Did you want it to backstall for the video?
....i have flown (in the past) that kite nearly a 100 times in my low wind area
If the SP21 had properly tuned lines the depower strap would be all out in 6-7kn...unless...
...it was flying extended with the 6 meter extensions..then a couple inches of depower is better
I do not call that back stalling...i call that a lull of below 5kn when the kite is at an angle in the sky
That kite will drop down with back lines slack when gravity takes over because too little wind in kite
6-7kn may be the wind at head height but up above in the sky it was like swiss cheese...full of holes
Wing tips will curl a bit when there is a lack of wind and the kite goes too high into wind window
No two light wind day is the exactly same
Some are powerful...some are gutless...and some are cheesy
I would not blame the kite but simply the extreme conditions you were trying to go in