By a non-staff user?
I tested on my test instance.
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I flag a post as an admin, thus it’s automatically hidden (and added as pending review).
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I log in with a non-staff user
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I open the direct post URL, either
/posts/1174.jsonor/posts/1174/raw -
I get a 404 error
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I can open these URLs as an admin, as expected
Did I do a step wrong to reproduce your issue, or did I misunderstand what you described?