I have a web app (built using the AGOL Web App Builder). The web app is shared with my organization (it is not public). I have added the Basemap widget, using four of ESRI's standard basemaps. In the Web App Builder interface, the thumbnail images for the basemaps correctly show up in the basemap widget.
The problem is that when I view the web app in a browser (not in the Web App Builder), the thumbnail images in the basemap widget do not appear. All I get is just four blank rectangles indicating where the thumbnails should load into. The functionality of the basemap widget is fine - when I click on one of the blank rectangles, it does in fact switch the basemap in the map display to the chosen basemap.
In my troubleshooting, I shared the web app with "everyone" (made it available to the public). With the app shared publicly, the thumbnail images do in fact appear in the basemap widget. So that's cool, but we don't want to share this app with everyone, just our organization. But if I share it with only our organization, then the thumbnail images in the basemap widget do not show up.
So it seems that this issue is related to some sort of permission/sharing problem. I have no idea how to solve this. Any ideas?