2. You are using slideout navigation (off canvas mennu) on mobile, so just remove :not(.slideout-navigation) from both code.
And you need to add nav-button class to the contact button in the menu that you assigned to the off-canvas menu as well, just like what you’ve done for the menu on the desktop.
I suspect there is a conflict when I’m on the page for which the menu item has nav-button applied. See what happens to the Contact Us menu item when you’re on the Contact page: