Same problem. It is interesting that it works in Map Viewer Classic, but in the new Map Viewer it is "locked" for some reason and cannot be reset.
When using MapViewer Classic, everything functions as it should, and you can zoom all the way down to "room" size. But even after I've saved my preferences, MapViewer's zoom level remains stuck at the "town" preset. Does anybody have any suggestions about how to use it? It's curious that although it may be reset in the older Map Viewer, it's "locked" in the newer one.
The same problem here. I have georeferenced a low-resolution map and published at the scale range 1:144,448 and 1:2,257 which is completely sufficient for viewing. It's annoying from a map viewers perspective that if just zooming a bit closer would make the map disappear. Even though in the layer settings I set the visible range to 'Room', in the (new) MapViewer the visible range below 'Building' is greyed out and when zoomed past Building the map is not visible.
A rough work-around I discovered was to open the map in the MapViewer Classic, then set the Visibility Scale to 'Room' -> Save and then again open in the MapViewer. Then it is possible to view the data also at a 'Room' level zoom.