My first APM flight...losing height!?

My first APM test!

The Talon flies really nicely in Stabalise mode but for some reason she loses height in RTL.
In fact the plane glides to home without any throttle.

For the life of my I cant seem to figure out why…
If you have some time would you be so kind to check my log and setup file attached?

What is your RTL altitude set to?