Quadplane QTUN.DAlt not tracking CMD.Alt

I’ve just started my first few test missions of my new quadplane, and it’s losing altitude throughout the mission until it touches down. It takes of to the right altitude, but descends through the waypoints. I have Q_ENABLE set to 2 so it should be in multicopter mode throughout the entire mission.

In the screenshot below, you can see that the target altitude is not even trying to reach the waypoint altitude - what’s up with that?

P.S I know the altitude controller tuning is pretty poor - I’ll get to that once I know why it’s not trying to maintain altitude>

Thanks everyone!