Interesting.. wish I knew. I just installed the beta firmware version 1.6.0 that Lorenz just uploaded with the takeoff fix. I programmed Take off and Mission on my Tx and in both (also in QGC), it recognizes that there's a "T" takeoff waypoint but I haven't actually tested because I'm in town.
Not sure if it will actually go to the "T" waypoint yet, but it should as it's the 1st WP of the mission.
When I either select takeoff or mission, armed of course, the motor revves up a bit and if you shake it as if you were attemtping to hand launch it goes to full throttle.
Perhaps LorenzMeier could explain in detail. We might have to test it out in 1.6.0 but seems that you already have but in 1.5.5. Keep in mind the takeoff for fixwing fix wasn't applied in 1.5.5.