All our data and maps are in NAD83 UTM. In portal we have a set of our own imagery and raster basemaps which are consumed within our web apps which is how our users interact with the data but we need to be able to also allow users to also use the default ESRI basemaps within the same web app. There are no issues with this in desktop applications (ArcMap and Pro) albeit at the penalty of projecting the basemap on the fly and the resulting antialiasing artifacts - both perfectly acceptable.
There is currently no way we can see to have basemaps with different spatial references in the basemap gallery widget and use them interchangeably in our web apps and having the app project the basemap on the fly without having to create a separate web app using the default WGS84 Web Mercator system. (Note: Switching over to the ESRI default WGS84 and tiling schema is not a solution as this is not suitable for our applications.) We would be fine with the performance penalty resulting from projecting these basemap on the fly and with the antialiasing issues resulting from mismatched tiling schema zoom levels. I understand the coordinate system is obtained from the basemap but surely there must be a way do do this since we have been doing in the desktop apps for decades.
In hopes to come up with a workaround we tried creating a basemap map in our default basemap gallery and first adding our custom NAD83 UTM basemap to the map in portal then adding a "layer from the web" and pointing it to the ESR World Imagery Service "World_Imagery (MapServer)". Although this works in the portal map and appears in the basemap gallery widget in the web app, when we attempt to add the basemap, the World Imagery service web layer does not display. Adicionality, simply adding a layer from the web to our web app using the add data widget allows us do add the layer, does not raise any errors, layer is even added to the legend BUT the layer is NOT displayed in the app.
Allow basemaps of different spatial references including existing ESRI basemaps to be added to basemap gallery and used interchangeably with other custom basemaps of varying spatial references in the Web App basemap gallery widget projecting the basemap to a spatial reference of our choosing.
In the interim, fix or a usable workaround solution would be much appreciated.