By the way - technically it transforms zoomed-out map into an empty map - I do not see anything for a moment at all. So probably you can move to another area and start to fill it for completely different area from the original one. And this completely doesn't make any sense in case of "extending/zooming out" of existing map (you cannot zoom-out empty maps). Also it keeps original map's name which proves it should be somehow based on the original one.
This issue is market as Works As Intended by I disagree. It's clearly a bug - when I place my map on Cartographer table it shows me in UI that my existing map will be extended without losing explored area (so it take my current map, makes it smaller and put some empty space around). So based on that I assumed it will keep discovered area. But it was not the case. So it should be UI fix to clearly indicate that you will lose discovered area, or logic should be fixed to keep it.
Note: this never happens while playing the game, but only when loading