The ArcGIS Online hosted application templates are unable to display custom basemaps from the Basemap Gallery widget when the custom basemaps are created using OGC Web Map Services (WMS).
Last Published: October 19, 2015ArcGIS Online
Bug ID Number
March 25, 2015
December 13, 2022
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.
"You can add additional basemaps to the gallery, however all basemaps added to the BasemapGallery must be in the same projection. If you are displaying basemaps from ArcGIS.com by setting then the additional basemaps must be in the Web Mercator projection."
Use ArcGIS Server services instead of OGC services to create custom basemaps.