menu_contextual_links() will always return a link to the MENU_DEFAULT_LOCAL_TASK, never the parent

Menu links never actually link to the full path of a MENU_DEFAULT_LOCAL_TASK, only its parent.

As a part of #1995620: [policy, no patch] Decide how to handle routes for MENU_DEFAULT_LOCAL_TASK, we're considering making the actual default local task path 404.

I'm not 100% sure if #914382: Contextual links incompatible with render cache changed this, but it certainly makes it harder to track down.
I believe that the db_select() in menu_contextual_links() is actually the issue.