I see you did not fasten the lower strap. That should make a helluva difference.
What do you mean? More specific, please, because as far as I know all the straps were fastened
take a look at the picture, it runs through the loop under the hook, I believe all mystic harnesses are equipped with it. Your video was too blurry, I can't say if you have mystic harness or only bought mystic spreader bar, that's the only explanation I have for why you don't have it there
edit: looking at mystic website, looks like there are some models, that might not have the strap. One I can see is Supporter model. I guess they dropped it because it's seat harness, so no way it would be riding up, but frankly I would not want a mystic harness without it (for the starters I'd never opt for seat though). I believe lots of companies use this extra strap nowadays, they make perfect sense.