This is somewhat difficult to explain, so here goes:
When you activate the overworld map, the viewer zooms in to 1.00 (despite the viewer saying the default 0.25) on the screen you viewed, then when you use the arrow keys to browse the map you get loads of junk/duplicate screens instead of white ones (the junk/duplicate screens are cleared when you explore those screens normally).
Also, when you stop using the arrow keys to scroll the map, the viewer zooms to 100% again on the screen which you originally viewed, and it moves the Link position cross.
So basically the overworld map viewer's pretty much unusable. 😞