Fonts missing in Linux (though available to other software)

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"This is a known problem: LibO gets confused when a font offers many weights or shapes"

Thanks for the quick answer and clue, RGB-es! That sounds a good one, and would explain why only 3 of the family appear in my Writer's drop-down font menu! I'll try to remove the ones I don't need and see if the needed one (Vollkorn Bold) show up. I'll post here the result of this new try.

Thanks a lot!

After removing all but Regular, Bold and Italic files from this family, and making sudo fc-cache -f -v only a generic "Vollkorn" shows up at the drop-down menu. And now, the text which was created in some documents with "Vollkorn Bold" appears in "Vollkorn" but with Bold applied. Font name appears at the text toolbar rendered with italic, meaning that it's a missing font. Still strange behaviour for me, but this will be an acceptable fix. ¡Muchas gracias, compañero! :-)

@touda
First, I suggest that whenever possible you get the fonts from the original source.
The fonts at 1001Fonts are v3.005 (2015-07-17), current fonts are v4.105 (2018-05-29).
There are now also ExtraBold and a Black fonts with italics.
So you may want use the newer fonts which have more features and fixes.
Second, these are great fonts; no issues in the fonts themselves.
All issues are LO.
Working 100% in LO on Windows.
Screen display, print output, and Export to PDF - all OK