This question is an exact duplicate of:
I posted a bounty on this question on May 27, intending to reward it to the person who had a highly upvoted answer that was accepted on May 19. The reason I chose from those available was
One or more of the answers is exemplary and worthy of an additional bounty.
When the time was up, I forgot to actually give the user the bounty. The system didn't automatically give it, even though it was the only answer there when I started the bounty and there are no deleted answers that were ever posted to the question.
I propose that, if the exemplary answer reward bounty reason is chosen and there was only one answer when the bounty was added and none were posted during it, the bounty should automatically be given in full to the pre-existing answer.