so I was adding some subtitles to my video and found out that,
some faces are displayed correctly while some aren't.
Any ideas why?
I can't reproduce that. What version of FCP and macOS?
Start by deleting preferences.
To delete preferences, hold down command and option while starting FCP X and click Delete Preferences.
The application will start in a default state, and open an "Untitled" library.
Reopen the library you were working on, set the preferences the way you like them, and try again.
Also: it is always a good idea to check your fonts.
Open Font Book (it's in your Applications folder) and let it validate all your fonts.