On MP and using ArduPlane, Flight Mode 6 is grayed out and defaults to Manual.
Screenshot
The value change be changed in the Config Tab and changing the value to 2 for stabilize, and the value sticks until the next change or save of Flight Mode settings. Then the value defaults back to manual.
My understanding is this was by design in MP as a safety measure. The rational being that every Plane should have Manual mode available from RC. Manual mode is often the only way to recover from a spin or a flight computer failure.
You can override it by going directly to the parameters and not using the MP mode setup GUI.
this is because in apm2 days that mode was always manual, ie had a hardware mux that could not be overridden. I don’t plan on changing it in the short term, as there is still a large apm2 user base.