Hi @CeBe ,
There is a limitation with the current release where we are blocking switching to a basemap if it is not in the same spatial reference as the map. This will be fixed in a future release.
If your basemap and your map are in the same spatial reference and therefore switching should not be blocked, we have seen an issue with packaging the basemap (tpk/tpkx) where you need to ensure the "Extent Sptial Reference" setting in the packaging tool (it should be near the bottom of the tool UI) is also using the same spatial reference as your tiles and your map.
We're sorry for the issues this is causing but hopefully that helps get you unstuck for now.
Chris