To resolve this issue:
- Quit Final Draft, if it's open;
- Go to your Applications folder and drag the Final Draft icon to the Trash;
- Download the installer for the version of Final Draft you currently own (the installers are available HERE);
- Run the installer;
- Restart your computer.
After the program has been reinstalled and the computer restarted, any italicized and / or bold text in your scripts should be formatted correctly.