But in reality, when I visited this protected question, it says: I'm not counting the association bonus, as I have already given all my association bonus points as bounty to a question. Is this a bug?
It seems that the bounty you offered adds $-100$ to what counts as "reputation earned on this site" for the purpose of answering protected questions. At least, that is my understanding based on the explanation given here:
- Stack Exchange doesn't allow me to answer protected questions because I gave away earned rep in bounties (Meta Stack Exchange; notice that it is tagged status-by-design) - you might also check some of the linked questions.
- Trouble understanding criterion to answer protected question (Meta MathOverflow - a detailed explanation is given in quid's answer)
Basically, so far you have (on MO):
- Started with 1 reputation point.
- Got the association bonus +100.
- Earned 10 reputation points from one upvote.
- Used 100 reputation points on a bounty.
That gives your current reputation, which is 11. But the association bonus does not count when checking whether a user can answer protected questions. So you have to add -100 and you're on -89. (In other words, you have to compare your current reputation with 110 reputation points, which is 10+100, i.e. the threshold for answering protected questions together with 100 points you were given as the association bonus.)