I just added a duplicate close vote + duplicate question to this question on Stack Overflow:
The vote was recorded:
But the question did NOT receive an auto-comment:
Does this answer your question? [question]
-- or --
Possible duplicate of [question]
The duplicate-target was also not listed in the list of Linked Questions.
Why wasn't the comment posted or the dupe-target linked?
I refreshed the page immediately after voting as duplicate and never saw a comment.