This bug is fixed now.

It came about as A result of the way render distance was programmed where the start and end were + and - the render distance of the cameras position with no capping so it went straight over the end of the map data when rendering out side of the map. If you players placing and breaking for tiles wasn't capped at the map border the player would have been able to modify these tiles outside of the map. Another thing this caused was the for map to repeat on the X axis.