Further investigation has reveled that it is, in fact, the font "Orbitron Bold" (taken from Muse's built-in Typekit browser) that is causing the issue. This might not necessarily be an issue with Muse so much as it is an issue with that particular font.
If it is the font, where should I go to report the bug to the author? It's a really nice font and I'd hate to see it remain unused because of such a small issue.