Almost every bounty i suffered from this change of rules. I read the rules carefully, did everything right but in the end i got %10 of what i earned. But there is no place to apply about this abuse so i let it go.
This is unacceptable.
However, normally most bounties will have a rule that states they can change the rules whenever. So that usually covers the bounty managers and the company.
That said, it's never fun to have your work go unappreciated with this kind of change.