Comentário oficial
We are sorry to hear you are having issues with your cached map. This is a known issue that has been a part of the system since the introduction of the offline basemap feature. It is caused by a known bug in how Apple draws overlays (the map tiles) when other drawing items are in place (lines). We cannot fix this.
Usually it can be fixed by zooming in our out some which will make it stop flashing. The issue is worst with smaller maps.
Since there is no way to actually fix this other than to remove the feature we figured it would be best to leave it in so people can use it even though it isn't perfect.