A common thing:
When opening "foreign" or longer unused libraries the problem arises that fonts or templates may be not found.
For missing fonts this font/font face will be replaced by the "default font" of the template and some parameters will be reset. The problem may arise that FCP might save this change - which is very annoying.
Unfortunately (for personal reasons) I can not update my old X-FX for Mojave as fast as I would love to do, I have decided to bring a part of it into a small stand-alone app: X-Library Check
This mini-app searches a library (.fcpbundle) for all fonts and external motion templates, lists them and warns if something is missing.
Application usage: Use Menu / Button to open a Library or drag Library to the App's Icon and wait a bit.
The result/report is saved as "Library Name" & "-check.txt" on the Finder level of the library.