I know this is based on:
If the index is created from previously compiled .img files, then the
same code page and sorting options (e.g. --code-page, --latin1) must
be used as were used to compile the individual map tiles.
- but it 's not the way it works. It is rather:
If the index is created from previously compiled .img files, then then only content in the
same code page and sorting options (e.g. --code-page, --latin1) as indicated in the mkgmap call will
be used. Searchable content from wrong codespace previously compiled .img files may be missing or broken
(but this does not matter if those .img files have no actual searchable content - e.g. transparent contourlines only maps)