Restricting menu parent items on content authoring form causes menu changes on edit

If you restrict parent items on the content authoring form using the Menu Settings options, you may see unanticipated results.

In our case, we have a main menubar using Primary Links and several sidebar menus which are implemented as blocks. For ease of use, I had restricted the parent item on the content authoring form to Primary Links. However, not all pages are meant to be included in Primary Links. Some pages are only listed from one or more sidebar menu blocks.

In this case, if a node that is normally included in a sidebar menu block is edited, the menu location of that node is forced into Primary Links, at the top level. This is quite unexpected, and took a long time to troubleshoot. Most significantly, this happens even if the user is not supposed to be able to edit menus at all!

If the menu parent on content authoring forms has been restricted, but the user does not enter a value into the Menu settings "Title" field, no change should be made when the form is submitted. And a content form change should not be accepted in violation of access rules, in any case.