I had this question:
All 3 responses were rated at 1 when the bounty expired. Nothing got selected as the "best answer", and I can't even manually select an answer.
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Once the bounty period is over, if no answer achieves a high enough score (+2 minimum) and is not checked by the question asker as the accepted answer, then it will not be able to win the bounty and the bounty will be lost by the OP.
Note: If there was a tie and the answers were at +2 then the first one would get the check.
An associated problem I've seen is when the automatically accepted answer doesn't actually answer the question.
I had a question where the best "answer" provided some food for thought and some ideas, but none of it panned out. Now, I have no problem with the bounty being assigned to that person...but it should be possible to unmark the answer so it's no longer accepted...because it doesn't actually solve the issue.
You didn't select an answer before the bounty expired, so you lost your chance. Also Since no answers had over two votes, no one was automatically awarded the best answer. This is not a bug.