Automatically Swicth to Altitude Mode From Offboard Mode

I use PX4_FMU_V6C v1.13.3 in UAV for offboard flight. While I use topic /mavros/setpoint_raw/local to control the UAV, the flight mode will automatically switch to altitude mode and the UAV flight unexpectlly. And a few seconds later, the flight mode will automatically switch back to offboard mode. I ensure this mode switch is not made by onboard computer or remote control.

Note that the area covered by blue means offboard mode, covered by yellow means altitude mode. Here is my detail log Flight Review - Quadrotor (

Is anyone meets the same problem? Any input would be greatly appreciated!