Safari not showing some of the fonts on certain website

Go step by step and test.

1. Startup in Safe Mode. https://support.apple.com/guide/mac-help/start-up-your-mac-in-safe-mode-mh21245/mac.  

    Quote:

    It also does a basic check of your startup disk, similar to using First Aid in Disk Utility. 

    And it deletes some system caches, including font caches and 

    the kernel cache, which are automatically created again as needed

2. Restore standard fonts

    https://support.apple.com/guide/font-book/restore-fonts-that-came-with-your-mac-fb34862/mac

3. Backup and  Reinstall macOS. https://support.apple.com/en-us/HT204904