Web AppBuilder is unable to display custom basemaps from the Basemap Gallery widget if the custom basemaps are created using Open Geospatial Consortium (OGC) map services.
Last Published: April 18, 2018ArcGIS Online
Bug ID Number
March 25, 2015
June 27, 2023
Operating System Version
7 64 Bit
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.
This is confirmed that it's a known limit of JSAPI 3.x unfortunately. Web AppBuilder is built upon JSAPI. The problem will rely on the API to fix.
Use ArcGIS for Server services instead of OGC services to create custom basemaps.