I notice that someone else is also having this problem on an Ubuntu system:
In my case, the problem showed up when I upgraded to LibreOffice Calc 6.1.2.1 (x64) on my desktop, running under Windows 10 Pro (ver. 1803). Calc’s FONT tab (under FORMAT menu) tells me that the fonts I’ve used for years without incident in my spreadsheets have not been installed; e.g.:
"This font has not been installed. The closest available font will be used."
But other programs on my system are not having this problem (e.g., InDesign is seeing the installed font just fine).
Unfortunately, this is a major issue for me.
If I can’t find a fix for it soon, I’ll have to revert to an older version of LibreOffice Calc.