Header lists posts instead of showing primary nav menu

Thanks to help from @Milo, I was able to figure it out. It had to do with the 'theme_location' => 'main_nav_primary' line in the wp_nav_menu function call in the header. In functions.php, the primary menu was just registered as primary so I had to change the function call to 'theme_location' => 'primary'.

I find it odd that the master site still served the correct menu with this discrepancy, but I’m going to assume there is some option or filter that made that happen and try to figure out which.