Before you join the project it is better to understand the rules they make. And for now, it is very natural when the manager changes the rules when the project is finished.
I agree with you, in most cases bounty managers leaves a caveat that the bounty rules could be adjusted or changed anytime. This caveat exonerates them from any future obligation which may arise from the adjustments made on the bounty. Its best for bounty hunters to be more informed on the bounty rules by frequently visiting the thread.
So as Bounty Hunter, we should be careful in participating campaign and not be confident enough to every project we've been a part off cause we can't tell that it will succeed or not.