There's at least a couple of occasions like
Where an answer consists wholly of someone else's external blog entry quoted in full. With no additional material.
This isn't plagiarism as proper attribution is given. WHen made community Wiki the answerer doesn't gain rep unfairly (unless there's been a change I missed) - so those are not issues.
However, if the answer does not contain an assertion such as "quoted with author's specific permission" - we can't know if this material is in breach of copyright or is posted contrary to the original author's wishes.
Is there, or ought there to be, some specific guidance on etiquette in these circumstances?
In this case the original material is available under a licence whose terms include
Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.