Here's something curious - sharing in case it helps anyone: when I add the layer to a basemap (R click>add to basemap) in new Map Viewer and go back to Map Viewer Classic, there's no more group to open, and only one pop-up that I have to turn off:
I guess you could say that it is inside a group, but it's essentially the "basemap" group from new Map Viewer. It doesn't open further.