I tried to open a WebMap, having a WebTiledLayer as basmap, in the Map Viewer application. It does not display anything in the map.
This can be tested using changing the query to group:"d6383ca17dae4c30836fccc7da3b80c4" and selecting Kartverkets GraatoneTileCache.
Finally managed to get this working by loading the qml dynamically.
Regards
Dinesh Mathur