I've had about 8-10 kilos plus water on it. There is maybe a little flex but barely noticeable. If you skip to 6 minutes there is some footage of descending col d'aubisqe, you can see I was confident in the setup - only flex that was an issue came from the (carbon) forks.
How many kilos have you had on it? I feels like mine starts to flex a little when I put my backpack with computer and school stuff on it.