I read the rules of implicit bounty award in the help:
If you do not award your bounty within 7 days (plus the grace period), the highest voted answer created after the bounty started with at least 2 upvotes will be awarded half the bounty amount. If there's no answer meeting that criteria, the bounty is not awarded to anyone.
If the bounty was started by the question owner, and the question owner accepts an answer during the bounty period, and the bounty expires without an explicit award – we assume the bounty owner liked the answer they accepted and award it the full bounty amount at the time of bounty expiration.
However they leave questions open:
What if the owner of highest voted answer is the person who offered the bounty, and there are no other answers with >= 2 votes?
What if the accepted answer has less votes than some other answer?
What if the accepted answer belongs to the person who offered the bounty?
I think the help should be updated to cover the answers, i.e. to explicitly state the priorities and relationships between the rules.