Plane motor spin when arming

We are building a quad Vtol. All works fine except that when we arm the vehicle the rear motor spins at low velocity (when we activate the transition it responds as expected). We’ve tried with generic quadplane vtol (connecting the motor at aux 5) and delta quad vtol (connecting the motor first at aux 3 and then at aux 5) and in all this cases it works exactly.

Any idea on how to solve it?


calibrate an ESC of pusher for late start.