@font-face fonts dont show up in wordpress after exporting a website to a wp theme

when I export a website as a wordpress theme, the custom fonts set on the site disappear. I’m not sure how to trace it back to where it gets lost, the theme editor shows the @font-face css, the font file is in the theme folders. To try to root out the problem, I’ve tried the same with an empty page that only contains the fonts, fontface rules and a single headline to test with, the same happens, the custom font gets lost somewhere when installing the theme to wordpress.
What am I doing wrong here?