I have an app that changes one the gray basemap so that it doesn't include the labels. It worked fine in 3.10 and also 3.11, but stopped working in 3.12 because it seems that "esriConfig.defaults.map.basemaps.gray" is gone. Looking into the "esri/config" object in the developer tools, there isn't no "basemaps" property at all.
Where did it went?
