calibre 7.26 here.
I have an eBook that has emebedded fonts. Two of the fonts are named Montserrat and one is named Monserrat Medium.
The error checker says to change the name to Monserrat medium for all of the fonts. I allow the error to be fixed. Then I run the error checker again and It's now asking for all fonts to be named Monserrat.
The two screen grabs show the the problem.
If I change the names to Monserrat and then manually change one of the fonts to "Monserrat Medium" then I no longer get an error.