So on the first question I flag it and it is asking me to select which question it is a duplicate of. I type the exact title of the question and it tells me it can't find any results! Yet I wrote the question's title exactly!
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It seems to be by design, not a bug.
It doesn't come up in the list because the question has no upvoted or accepted answers (classified as an "unanswered" question) and is therefore ineligible to be a duplicate target.
As Andrew Barber noted, you can just paste the URL into the box (which would tell you the reason outlined above):