Start mission via MavLink no QGC connected

I tried to upload and start missions through Mavlink but encountered some issues.
I am testing the latest version (1.12) running HITL on a Linux machine.

I created a mission on QGC with three waypoints and uploaded it to the flight controller. I am trying to take off and start the mission with an app that I wrote using Mavlink. I managed to takeoff, but when I try to start the mission using MAV_CMD_MISSION_START, the drone stayed on TAKEOFF mode as soon as I opened QGC, and the drone switched to MISSION mode and started the auto mission.
What can cause that?