S Multiplier is the smoothing multiplier value.
My problem is that when I move closer in the Z axis, this number changes to >1.0, the number becomes boxed in yellow and pitch and yaw become 'frozen'.
That's what it's for.
The zoom smoothing multiplier increases smoothing to stabilise view, when you zoom in. This makes your zoomed view more stable.
when it's yellow pitch and yaw are inoperative regardless of the aboslute value of the number.
Even when you reduce this value to 1?