I HATE VOXELMAP AND MAPWRITER and why cant micdoodle8 fix this?I believe the problem is on Rei's. It has been reported dozens of times (e.g. link), it's a very trivial bug (link). I don't believe that it will be fixed so soon since Rei's stopped updating his topic at MCForums and now only posts on Japanese forums. Try Voxelmap or MapWriter.
I'm looking for replacements but it's not easy to find something as functional as Rei's minimap. :<