This bug means that you have another addon interfering. GatherMate is only able to tell your current location in the world if the world map is not open to another zone that is different from your current location.
Example: You are in Zuldrak. If your map is open to Zuldrak, or not open, it works. If your map is open to Grizzly Hills, GatherMate no longer can tell your coordinates and location in Zuldrak (because your location disappears from the map) and stops updating the minimap.
This bug can also occur if you have another addon that is temporarily or permanently modifying which zone the map is viewing in the background, even if the map isn't open it can be set to show a certain zone which might nto be the current zone. By default, the default UI always switches the map to the current zone when you change zones (even subzones).
From the sound of your bug, I would conclude that some addon you have is scanning your map zones and changing which map the world map is currently viewing when the lance is equipped. (Maybe an addon that tries to generate macros automatically?)