As an admin, editing a project when you are not the group owner will remove it from the group namespace.
Created by: yuters
The issue was previously resolved here #2375 (closed)
But now on the latest revision, an admin cannot see a group of which he's not the owner on the namespace dropdown. (in project -> edit and Add new project)
If even an admin can't see another's group namespace, I can still edit the project (for example, to change the default branch) and it will automatically move the group to the global namespace after that (because the group is not in the "transfer" dropdown).