Important Update: There is a bug in QGC 3.4 which can cause the altitudes in mission items to sent to the vehicle as 0 when the ui still shows a good value. The easiest way to reproduce this is to:
* Add the first Takeoff item to a mission
* Change the Takeoff item to a Waypoint
* Add a new waypoint item
All subsequently added waypoint items will show in the UI with a reasonable altitude but will be sent to the vehicle as 0 height which can cause a crash.
Standard mission creation where you add the first item which is added as a Takeoff (leave it as Takeoff) and then add subsequent waypoints is working fine.
Issue details are here: https://github.com/mavlink/qgroundcontrol/issues/6823. A new 3.4.2 which fixes this will be available in a few hours.