I suspect this may have to do with the Map Selection Area Polygon (0x004a) and/or the Map Coverage Polygon (0x004b). The OSM samples above were compiled as non-transparent maps, but the gray background is the same color as the NoMap color in MapSource 6.16.1. So it looks like 6.16.1 may be treating the OSM maps as if they had no background. The light beige in the lower right of the following image is the standard background color.

Also note the yellow map selection polygon highlight is offset from the actual map bounds. This is a routable map, so the actual limits of the map selected are the bottom of the horizontal contours and the right side of the vertical contours, but the gray background aligns with the map selection area. The selection polygon offset has no effect on the routing.