Basemaps do not load in the Web AppBuilder app if they have a different tiling scheme than the default basemap of the web map.
Last Published: March 17, 2017ArcGIS Online
Bug ID Number
January 21, 2015
September 22, 2022
After review by the development team, it has been determined that this issue is related to a known limitation with the software that lies outside of Esri's control. The issue's Additional Information section may contain further explanation.
It's a known limitation. The basemaps with different tiling schemes from the current basemap are now excluded from the widget. It'll be documented.