That is because ‘watched’ things only watch for files of which there are plugins for, which is generally just js, css, and html (templates). You probably want to add that path as an ‘asset’ directory and that ‘might’ work. What would work fine though is to make a brunch plugin that you can specify font paths into and it can copy the files over to the final static directory. I’ve been writing a few brunch plugins lately so I might be able to do soon (probably weekend) if you remind me enough?
Also do note, the ‘usual’ way of handling fonts is having an npm task (in the package.json file) that copies them from the npm module you depended on into your own work tree directly. That is what I do, although I do not like it, I would prefer a plugin too…