We have created a custom basemap using the Vector Tile Style Editor.
This tile layer was then added and saved as a web map and shared with our custom basemap gallery group.
ZacharyHart_0-1701284413777.png
Our AGO site is configured to use this as the default basemap in or organization.
As you can see, there are only 2 layers in this web map: the vector tile layer and hillshade:
ZacharyHart_1-1701284517072.png
If I then 'Open in Map Viewer', the layers are as expected:
ZacharyHart_2-1701284653681.png
However, if I create a new/blank/empty Web Map, the custom basemap layers load as expected, but the World Topographic map is added as well. So now I have two overlapping basemaps and need to manually remove the world topo one.
ZacharyHart_3-1701284774747.png
The same behavior is observed in ArcGIS Pro when creating a new map:
ZacharyHart_4-1701284917385.png
Why is the Esri basemap loading by default in addition to our custom basemap when our AGO site is configured to use the custom basemap as the default?
ZacharyHart_5-1701285055283.png