It does indeed appear that your bounty was refunded as part of the question being deleted.

The answer is at https://meta.stackexchange.com/questions/145329/when-a-question-that-had-bounties-is-deleted-are-they-automatically-refunded which shows bounties being refunded. This is a feature, not necessarily a bug.

If you were to place a bounty that was successful and/or got an upvoted answer you would not be able to get the bounty refunded after a question is deleted. 

Bear in mind that any upvoted answers will block a question from being deleted and as a result will block this kind of abuse.

It is not an "infinite bounties" hack, more it is a refund of wasted points.

In this particular case you lost the points from the two upvotes your question had, and got your points lost as a bounty back.

There is also a question on Meta Stackechange [How does the bounty system work?](https://meta.stackexchange.com/questions/16065/how-does-the-bounty-system-work) 

> ### What happens if a question where I awarded a bounty later gets deleted or migrated?
>
>If a question on which you have started or awarded a bounty on later gets deleted, then in most cases, your reputation is refunded. It can take a few minutes for this to happen. If the question is later undeleted or migration is rejected, the reputation is deducted again. 
>
>There is one exception to the above rule, however: if the bounty was awarded to an answer, and the author gets to retain their reputation upon deletion (i.e. their answer earned a score of at least 3 and was visible for at least 60 days), the reputation is not refunded to the bounty starter.