there are 2 weird things in your maps. First is a subfile SRT, which
doesn't belong to any tile or index. It is present in both files, bad
and good img. It is probably ignored by GPS.
Maybe mkgmap writes SRT even if MDR is not present?
Second distinction is that bad img is missing MPS subfile, which is kind
of directory with names of combined img. This could be the reason for
different name in GPS, but I think the map should still work correctly.
Again, maybe there is some kind of limit of tiles, that mkgmap
remembers, when creating MPS subfile?
So IMHO the problem is with combining your map into single gmapsupp.img.
You could install map for BaseCamp and then use MapInstall for creating
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