I think the altitude should read a negative number if you fly below the Home Point, but the Spark will continue to fly.
You may have to bump your maximum altitude up if you launch from the valley floor and want to rise up over a ridge that is higher than the limit the Spark is set for.
The Spark will not lose height on its own. If you lower the Spark towards the valley, your altitude display will show a negative value i.e. the take-off point is taken as the origin for altitude display on the Go4 app.