Motion title font keeps changing back to default in Final Cut Pro

Ok... as a general rule... any parameter that is part of a Rig cannot be changed in FCPX (even if you can change it temporarily in the storyline).

Specifically, Lower Third Basic has the font it uses (Helvetica Neue Bold) in a Pop up Menu widget (Title Style in particular). All you need to do is right click on that parameter and Remove from Widget Title Style. Double check the font of your text to make sure it is still what you want. Save it again.

Back in FCPX, select the title in the storyline that breaks and double click on the newly saved version of it. It should update the storyline title with the updated version without having to re-do the text you've already applied.

There's actually quite a lot that has to do with Motion that is only learned by experience — and/or people sharing what they've been able to find out by trial and error. Some of these kinds of details don't appear in the documentation (to my knowledge). Don't trash your Motion skills for this! And, from here on out, any time you customize a template and it doesn't work right away— you'll know exactly where to look first!