It looks like the Default font family is taken from the OS list of fallback font families. Wonder how it could be switched to the fallback monospace font families?
I seem to remember reading something several years ago. Is there a handler for fallback of style (serif, sans, monospace)? That handler would select whichever was the 1st matching style fontface that found to exist in the OS’s font families list.
Note: I started from the 5.2 Highlights which lists the addition of Strikethough & Superscript/Subscript to the Note Editor. That PR linked to the changed source code files.