RTL mission item changes from Terrain to Absolute on writing causing Controlled Flight Into Terrain (CFIT)

[GrahamDyer]

A strange anomaly while planning and flying a VTOL mission in SITL which results in the aircraft flying into the ground.

Dangerous to say the least if this is the case in real flight.

Loaded SITL > quadplane > Plane using the latest Mission Planner. I then created a mission (attached as a .txt file, .waypoints file wasn't accepted as a file type),

I upload the mission with all the waypoints designated "Terrain" and the last waypoint being RTL also as "terrain" but when I read the mission back RTL changes to "Absolute". Terrain following is enabled however when I fly the mission the aircraft flies into the ground.

See the attached video. It clearly shows the "Terrain" turning into "Absolute".

Will this happen in real flight?

Mission.Planner.anomaly.mp4

Flight Plan Anomaly.txt

[tridge]

The RETURN_TO_LAUNCH mission item just changes mode, it is a command that doesn't have location fields (ie. "has_location" is false for this mission command). There is no lat/lon/alt or frame stored in ArduPilot for commands like this.
It would be better for MissionPlanner to display "N/A" for not applicable.
If you want RTL to follow terrain then please read the documentation for the TERRAIN_FOLLOW parameter.