I just experienced the exact symptoms described in this previously fixed bug on a brand new question on Stack Overflowbrand new question on Stack Overflow.
Note that while the symptoms were the same (the dupe notice disappearing), it happened under slightly different circumstances, in that it was a live, active question with other people editing their own answers, rather than opening a new tab to an old, already closed question.
Reconfirmed the bug on this question. The answer got edited, reloading it caused the notice to disappear. I used this search to watch for likely instances of it occurring. Note at the time of writing, that question has 2 reopen votes, but it is/was a duplicate at the time of refreshing the answer.
Update
Seems that it's not just happening on duplicates. I've just witnessed the same behaviour on a question with a Bounty notification banner - reloading a edited answer also causes the bounty notification to disappear.