Menu deleting items, not saving theme location

When I edit my menu and click save, the last 3 entire menu items are deleted, and the theme location of the menu is unchecked. This is the exact same problem as this previous thread: https://hb-themes.com/forum/all/topic/i-tried-to-add-one-menu-to-my-existing-site-and-after-saving-menu-is-destroyed/#post-57824 (the solution was not posted publicly). This is a huge issue, and obviously a problem that has been posted before. I appreciate your help getting this resolved as quickly as possible.

The theme is up to date, as is WordPress.

Thanks!