HELP! Runway Takeoff Disarm problem

Running in mission, all parameters passed, arm the aircraft- it arms itself and begins the takeoff but after approx 2 seconds the aircraft disarms and rolls to a stop. Why is this? (working with an X-UAV Talon)

takeoff runway or hand throw?

Is the parameter COM_DISARM_PRFLT set to negative value?
The vehicle could be disarmed automatically if the value is positive.(https://docs.px4.io/master/en/advanced_config/parameter_reference.html#COM_DISARM_PRFLT)