It occured disarmed after I arm throttle

When I type this command below:
```
mode guided
arm throttle
takeoff 2
```
I got the problem below.
```
STABILIZE> Mode STABILIZE
arm throttle
STABILIZE> APM: Arming motors
Got MAVLink msg: COMMAND_ACK {command : 400, result : 0}
ARMED
takeoff 2
STABILIZE> Take Off started
Got MAVLink msg: COMMAND_ACK {command : 22, result : 4}
takeoffAPM: Disarming motors
```

Reproduce the reported `mode guided`, `arm throttle`, and `takeoff 2` sequence in the repository's ArduPilot Gazebo SITL setup, then inspect the takeoff and disarming behavior around the reported command acknowledgements. Done means the cause of the immediate disarm is identified and the command sequence behaves as expected, with a regression check for this scenario.

Written by the indexing model from the issue text.