In ArcGIS Online, custom basemaps created using the server image service with projections other than the spatial reference 3857/102100 are disabled in a local scene basemap gallery.
This is expected behavior based on the Scene Viewer's basemap requirement documentation, https://doc.arcgis.com/en/arcgis-online/create-maps/choose-global-local-scene.htm#ESRI_SECTION2_F721D1167B5E4F27851254CEBADD531C.