I've seen a lot of posts about various seat post clamp design issues where the seat post height slips but this is the first one I've heard about the seat angle not holding. I don't have any advice but if it is subject to suddenly rotating to the point it causes crashes, I would consider applying epoxy to lock it in place. Not a great solution since it would prevent ever making adjustments but I wouldn't feel good about selling a bike with that problem so if that would make it safe to ride, maybe the best solution.
Would be interested to hear about your complaint and whether they respond at all. Hope it gets resolved.