The full story is , : its a feature source for displaying QGIS features in osgEarth, the features are created in QgsGlobeFeatureSource::getFeature resp. QgsGlobeFeatureUtils::featureFromQgsFeature. Since QGIS also supports features with 3D coordinates, I'm trying to get these to display as full 3D objects in osgEarth as well (instead of merely extruded 2D geometries). As a first experiment, I just changed QgsGlobeFeatureUtils::geometryFromQgsGeometry to always return the geometry I posted in the initial post.
So in short, as soon as I add an altitude symbol, regardless of configuration of that symbol, the geometry is rendered as garbled patches. Actually does an altitude symbol even make sense for such a geometry? I was hoping to have an option to offset the Z coordinates by the terrain height, but perhaps I just need to require that the models have the correct, absolute Z coordinates, and omit the altitude symbol altogether?