As Roland says, I believe this behavior is intended. In altitude-controlled modes (e.g., alt hold, loiter, pos hold, etc.), the throttle stick commands a climb rate, rather than a throttle or power setting. When you're on the ground, a stick below the half-stick deadzone will keep the motors at MOT_SPIN_ARM until you raise the stick above the deadzone, where the FC will then attempt to match the commanded climb rate by increasing throttle.
If you want the motors to spin while armed on the ground, set MOT_SPIN_ARM to > 0. MOT_SPIN_MIN is used as a minimum motor throttle when the copter is in flight and shouldn't come into play when you're on the ground. If you want the motors to respond to the throttle stick below the deadzone while landed, I think you will need to use a non altitude-controlled mode.
I honestly don't recall if this is the same behavior as Copter 3.3, so it might be different than what you're used to.