Below, if mFGFilled is not null, but the fillDirection doesn't match the direction of the UIFilledSprite, the thumb won't move. I think that else if should just be an else condition; or there should be an else that fires off a warning/error. Took me a while to find out why the thumb sprite wasn't moving around when changing the sliderValue from the inspector.
if (mFGFilled != null)
{
if (mFGFilled.fillDirection == UIFilledSprite.FillDirection.Horizontal)
{
pos.x = mFGFilled.invert ? fullSize.x - scale.x : scale.x;
}
else if (mFGFilled.fillDirection == UIFilledSprite.FillDirection.Vertical)
{
pos.y = mFGFilled.invert ? fullSize.y - scale.y : scale.y;
}
}