According to the privileges page, when a user starts a bounty,
The question is immediately bumped to the top of the active question list.
I have three active bounties on MSO at the moment, and I don't think a single one of them caused its question to get bumped to the front page when I offered it. The
/faq entry and faq entry don't mention this behavior.
I do remember getting bumps for past bounties and was expecting bumps for these recent bounties. Was that feature disabled?