I don’t quite understand why you’re expecting MOT_SPIN_ARM to be the lower value.
As I know the primary goal of AirMode is to always maintain vehicle attitude. So how it will be possible with MOT_SPIN_ARM throttle value? It is allowed for MOT_SPIN_ARM to be zero.
MOT_SPIN_MIN, which by definition is the “point at which the thrust starts”
This could mean not “when vehicle start lifts off” but “props are able to change/maintain desired pitch and roll”
So why can’t you just decrease your MOT_SPIN_MIN?