The coordinate system for a map makes sense only if all attached layers (i.e., the layers composing the map) are defined with a proper coordinate system. Adding to the map even a single layer without a proper coordinate system turns Viewer.CS to off.
The coordinate system of the map in the Viewer is used to reproject the map, provide proper scale, etc. Even a single layer without a CS makes all computations unpredictable. In such situation, therefore, the software simply swiches off the whole coordinate system.