Auto flights to negative relative altitudes

I get an error message If I try and upload waypoints with a negative relative altitude.

The message implies there is some parameter or constraint that can be adjusted but I’ve not been able to find anything in the wiki.

Has anyone managed to get this to work so you can fly lower than your Take off spot?

This also happens to me … I set the various waypoints at a flight height of 20 meters … take-off point set at 10 meters high … when it leaves to reach the next waypoint at a height of 20 meters. … on the way to get to this from the hud I am told that at a certain point it goes to a negative altitude of - 1 meter … and I don’t understand why.