Beginning September 1, 2022, vector basemaps will no longer work correctly in applications using ArcGIS Runtime SDKs 100.5 and earlier. The application itself will continue to work, but the basemaps will not display correctly. You will see gaps, some of them large, in the basemap.
Developers are strongly encouraged to move their applications to a newer version of the ArcGIS Runtime SDKs before September 1, 2022. If that is not possible, an alternative is to use raster basemaps, which will require some code changes. Note that most raster basemaps are in mature support and are no longer updated.