The lens should not collapse beyond 55mm position, unless the locking button is pressed. To extend the lens you have to hold the button, twist the lens and release the button when you reach the working range. The button should pop up. Then you shouldn't be able to retract the lens without pushing the locking mechanism button. Of course, for setting the desired focal length you don't have to push the button, you operate it like any other zoom lens.
I also find it strange that the lens collapses under camera weight - while focusing ring works with minimal friction, zoom ring is quite stiff and not as smooth working as I wished it to be. But I admit, I didn't try to transport my setup standing vertically on the lens.
When I hold the camera in my hand (body or lens barrel) or it's dangling on a strap, I didn't notice any changes in set focal length, never.
I'm sorry if it's all evident, I don't want to sound like Cpt. Obvious.