The problem that you've encountered is most likely a result of corrupted font duplicates and corrupted font databases – in cases when it occurs, FCP displays random or blank characters. Please follow the below instructions to fix it.
Open the Font Book.app (you can find it with CMD+SPACE)
Go to All Fonts
Resolve fonts duplicates via the top menu File>Resolve Duplicate
Empty the trash
Reinstall the plugin with mInstaller and check if the issue persist
Next, open the Terminal.app (you'll find it in your Applications > Utilities folder), copy-paste the following command, and press Enter (be sure to type in your admin password when prompted):
sudo atsutil databases -remove
Once the command has been executed, restart your Mac.
Lastly, you'll need to delete the generated previews (only if you've got Background Render turned on - Final Cut can store a pre-render of the incorrectly displayed font) via the “File” > “Delete Generated Library Files” menu (see attached screenshot - link at the bottom) - after that, the issue should be gone. Here's the tutorial for clearing FCP render files: https://youtu.be/j7_aWuVJbW8