This issue is called FOUT (Flash of Unstyled Text) and you can read more on this here: https://webdesign.tutsplus.com/articles/quick-tip-avoid-fout-by-adding-a-web-font-preloader–webdesign-8287
You can fix this by adding a bit of custom code to your site — @cyberdave explains this more here: Flash of unstyled text - fallback fonts loads initially then switches to google font