This is a subtlety that we probably need to add a warning for. The mission data for the flight path is all that is synced. The offline basemap data is not saved as part of that sync. For any area that you need basemap tiles for a synced mission you will need to hit the Unlock button for that mission and save it again. Or create a new mission and save it for the same place. This will trigger the tiles to be downloaded for that specific area.