wp_nav_menu primary menu selection in admin not saving

I am editing an ecommerce theme and have activated the wp_nav_menu option in the functions.php file. This was working properly until it just seemingly randomly reverted to the default menu option. What I have been able to figure out is: in the admin section, it will not save the selection for the primary menu; I can select it and click save, and then the selection is blank whenever I go to another admin page and back to the Menus page. It will even save other menus I add, and retains all the individual menu options, but it just won’t save the selection for Primary Menu.

I did have this was working properly before, and I have only been editing the theme files, so I am unsure of what I could have broken. Any help on what might be going wrong and how to fix it would be much appreciated.

(link to staging area: https://thinklinkmarketing.com/care1stwholesale/)