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Since the book was in Kobo Plus, I picked it up to have an original to look at.
calibre's e-book editor gave multiple errors. I've added images of the ones related to the fonts. The @font-face entries all refer to Montserrat while one of the fonts internal name is Montserrat Medium. This appears to be your original error and is an error in the ebook not in the error checker. If I edited the font weight 500 entry to "Montserrat Medium", that error went away. Some of the other errors reported such as font/ttf being an error are indeed errors in the error checker since ePub3 allows that media-type for TTF fonts.
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