(I wasn't sure if this was a code, user interface or miscellaneous component!)
Whenever a page is edited, the primary link menu title attribute gets reset (nulled) when the edits are saved. Disabling the Menu Attributes Module stops this behavior.
After a page edit, you must manually reset the menu description in the Site building | menus | primary links page.
It would be nice if the menu description (title attribute was left alone by the Menu Attributes Module.