FID 0 is a kind of depreciated value, this value is used by maps, that do not belong to "Family" but "Products". Both names comes from Mapsurce registry keys on PC. I'm not sure, how current gps treat "Products", maybe just as you observe.

For this maps you can change FID/PID value with gmaptool, but in a bit complicated way. You need to split img into maps (not "files for Mapsource", but "single maps") and than join all this maps giving mapset name and new FID/PID numbers in gmaptool join options.