On my site I have discovered that the 404 Redirected plugin causing the issue with shortlink redirects.
When it is activated,
?p= generates a 404 when the ID is for a PAGE (as opposed to a POST), if the Automatic Redirects option is not activated.
If the Automatic Redirect option is activated, then it will redirect to the home page. Either way, it stops the shortlink for PAGES from working.
When it is activated I have to use the shortlink
/?page_id=## but this makes it a real nuisance for setting up Apache redirects. So for now I have to make do without 404 Redirected. Would love to see this fixed.