The meanings of the flags of offboard_control_mode is a bit misleading as the position controller module (which control the tracking of position AND velocity setpoints) is enabled if (not only if) offboard_control_mode has velocity or position enabled.
Therefore just setting velocity=true and postion=false does not stop the position controller from controlling both position and velocity.
You have to set to nan the position setpoints.
In your case the vehicle stops at -0.5 as a result of combining a 0 z-reference and a -5 vertical ascend.
