I have not noticed this effect, maybe you can check with gmaptool if the MS flag in the header is set to 0 or is still 1. If this is set to zero, you can view the img in Basecamp. Maybe you can also see it in combination with gmapbmap?
See http://www.mail-archive.com/mkgmap-dev@lists.mkgmap.org.uk/msg10457.html

Another tool to reset the Mapsource flag is with imgmod from Javawa, see the link in my message above.

Edit: just checked out some maps of raumbezug (gmapsupp from Hamburg, Belgium) and noticed they set the Ms flag to 0 and it shows fine in Basecamp.