I have the same behavior as @jamescooper1
My MOT_SPIN_ARM is 0.0 and MOT_SPIN_MIN is 0.15
So I have my copter disabled on the ground and want to take off in AltHold mode. Here are my steps:
1. Throttle to minimum, switch to AltHold
2. Push safety button and arm (no motor spin becasue of MOT_SPIN_ARM is zero)
3. Increase throttle stick smoothly: 10%...20%...40% - Nothing... Motors are not starts.
4. After throttle above 50% - motors starts and copter takes off.
I mean it looks weird that motors not spinning at MOT_SPIN_MIN value right after throttle stick above minimum value.