Ed_slv, I just made a small mapset with Mapsource v6.16.3, containing some western tiles of Spain (Portugese side), no problem at all. I created my own Mapsource version from the Topo España v5 Pro img with GMapTool.
There must be something wrong with either Mapsource or your map.

Since I want to keep the index search working I don't create mapsets with Mapsource very often, I only change the typ with my own version (also with GMapTool) and put it on one of my large SD cards.