Are you wrapping the content (words or line or …) that you want in your custom font in a or other tag?
I have several publicly available decks that use custom fonts, some extensively, and they are all working fine. If it will help, download my Vocabulary from Elements of New Testament Greek, 3rd edition and look at the cards. You will see that I wrap the paragraph, in which I want a specific Greek font to be used, in
tags. The greek class is then defined on the Styling tab of the Card Type (“.greek”) to use the specific font.
If you would like a different example, in which lots of specific fonts used (using mostly the tag), then look at my Byzantine Music group of decks.
It might be easier for you to see a working example.