I have no idea what’s going on, I’ve never seen that before. It could possibly be a plugin or setting…..or maybe you have custom code added somewhere. Double check any code that you’ve added in your functions file or any other place you’ve added code to customize your theme.
When I look at the link, I noticed the code for the nav itself doesn’t show on the page at all. So there is definitely something preventing the code from showing on that particular page. It could be some type of conditional code somewhere if it’s not a plugin.