Found the solution, or rather, someone in the Thesis forum told me:
“A page named “menu” will get a custom body class of “menu” which is probably an issue as the navigation menus also get this class. If you can get away with naming the page “Our Menu” or anything else that is not just “menu” in terms of the body class assigned, that should resolve the problem. “
I tested it in another site, he’s right. Case solved.