Yes, this is a known mkgmap issue. It seems the gmapsupp img’s become transparent when viewed in Basecamp (if they are read from SD card or GPS unit). See this screenshot: http://forum.gps.nl/download/file.php?id=410 and read (in Dutch, so use with google translate) this thread http://forum.gps.nl/viewtopic.php?f=109&t=36229 or this page: http://www.javawa.nl/gpskaart.html

There is a workaround and that involves replacing the Basecamp “Global Application Basemap v2.gmap” (which does not contain anything interesting at all) by an empty one. This map is located in C:\Program Files\Garmin\BaseCamp (or another folder is also possible).