Adding our custom base map to an Experience Builder Map renders background black...

[Lake_Country_GIS]

I have a an ArcGIS Online map containing a custom base map:

https://arcg.is/1nKnin

In the map, the rendering is fine.

Lake_Country_GIS_1-1653674221590.png

If I add this map to an Experience Builder project, the background renders black.

Lake_Country_GIS_0-1653674207430.png

Any idea why?

Thanks in advance,

Mark