Probably, it is their choice to fix the issue and collect. An unresolved issue in the agreed timeline automatically refunds the money. Maintainer has the option to request more time. Donors can accept or reject. Ideally multiple people would sponsor an issue, so some percentage of funding would back out as deadlines are missed, but not all funding. Incentive is more delay, less reward.

What happens here is inevitably someone puts up $5 and harasses the maintainer for not taking the bounty.

They already do while putting up $0. Nothing really changes there. Also, maybe money should be refunded if maintainer does not accept in a reasonable time after funding goal is reached. Maintainer can always reject sooner. Maintainer has final word.

People feel a lot more justified to harass maintainers if they offer money, and do not usually understand that a small token amount is not really worth taking :(