Why should the map show anything outside of those constraints?
If you want a theoretically infinite map, just leave both options empty when creating a map. If so, the Minimap will show new chunks.
If you add tiles outside of the map width and height, the map should show those tiles. Also, this is ...

Note: This can also be considered intended. However, it could also be a bug. That's why I posted it here. It would be nice if the expected result is what happened over the real result.

Bug:
The minimap doesn't load any more chunks past the map width and height set at the beginning of creating a ...