Is it just me or is there a bug that you can't jump to fonts alphabetically by typing letters anymore? This is when choosing fonts within a title. I thought you used to be able to at least type a first letter to jump through the list to that section alphabetically.
Just a guess here, but you might have a corrupt font. You can look for that with the fontbook app (yellow exclamation sign next to problematic fonts) and you can also check for enabled duplicates and then disable or resolve any font issues. If you have another fontmanager installed aside from the standard OSX Fontbook, that has been known to pose various issues with other applications.
I've had the same issue in the past, couldn't figure out a fix for it back then. Just checked and looks like it's working now, but I did change computers lately so I'm not sure if that fixed it.
The whole font tool is kind of a mystery for me in FCPX. Besides the issue I previously had, it's confusing to me how pressing a letter often takes you to the first font starting with that letter, but sometimes it's a random font, like the fifth font starting with that letter. Not sure if that's also a bug.