I think it is natural, if the project really does not achieve success. And that is part of the risk that is in the bounty. But don't think that failure will be continuous, rest assured and all that will definitely have results. Of course the project manager certainly hopes that all will succeed according to plan.
Keep the spirit!!!
I beg to differ on this mate. Its not in anyway natural for projects to just back out of their own stated payments for bounty. Bounty hunters should not be treated as working for free cos they also invest in their time, data and energy into working for the project. If a project can not pay for bounty, it should be well stated from the onset.