Is it normal for a siphon in a toilet cistern to tilt to the side when the toilet is flushed?
That's going to depend on the type of valve and the material it's made of. In your video, the valve moves because of the force to lift the flapper. That's normal. You just have to make sure the valve is tightened to the tank correctly so water doesn't leak out each time you flush and the valve moves. There i no reason to try and design a method to secure it, just install it correctly.