I would start with trying to revert your CSS changes and seeing if there's any difference. Please keep in mind that Joplin internal styling may change over time, meaning that you need to keep updating your custom CSS to match the current version of the application.
Turns out *{ font-size: 14px !important } in userchrome.css is causing it somehow. But I’m at a loss exactly how. I conceptually understand CSS at a high level, but I’ve not deeply studied it and my understanding is far from complete in the details, particularly how selectors and attributes interact, etc. Plus I don’t know what Joplin exposes.
When I first added that – almost certainly scarfed from the forum here – it worked (i.e., made the UI fonts larger) without the note title side effect. Why that changed I don’t know. Commenting it out makes all the UI fonts smaller except the note title, which gets larger. As a workaround I’ve set it to 15px, a compromise.