If you switch between the other categories next to it, it reappears. I know it's not a proper fix, but it's better than starting over.
When I add multiple types of related pages (e.g. related characters and related objects) to any page on the wiki, the "submit" button disappears when I switch tabs. For example, if I add some related characters to Cybertron, then click "concepts" to add some of those, the "submit" bar at the bottom of the screen disappears. When I go back to "characters" it comes back.
Win 7 / Chrome