I have a different problem with the UISlider. I set my slider.value to 0.5f in code, before it is displayed for the first time. However, the first time I try to drag the thumb when running the game, it jumps to the 1.0f position. Subsequent drags of the thumb, and if I click anywhere on the background on the first or subsequent times, work fine.
In editor mode, I have the thumb positioned at the midpoint and the value set to 0.5f. It makes no difference - for some reason, the UISlider wants the thumb to start at the 1.0f position when dragging it for the first time.