I have experienced the same issue with blended basemaps appearing correctly in the web Map Viewer, but one basemap being completely hidden in the mobile app. I would be eager to hear a solution; for now I have resorted to increasing the transparency of one layer.
This is the only documentation I can find on this issue. It bums me out because color blending with the basemap easily provides the best looking symbology for the field crew's use. I hope this can be resolved at some point.
