Many bounties have already says it all in their rules and regulations that 'THE BOUNTY MANAGER HAS THE RIGHT TO CHANGE RULES AT ANY TIME' when you see something like that always know that anything can happen ,the one way to avoid this is if there is no such rules on the bounty thread.
Bounty hunters still find it very hard to read,they care less about rules and go for the good looking bounty allocation, please have patience and go through rules and regulations ,most times its not always the bounty managers fault.
The problem most of often in this situation was the community who joined in the project campaign, they never read first the rules and policy on which the project has, so in the end they complaint after the token sales. So, my advise is that always take a screenshot if you see some project about their rules so in the end you have proof of evidence to complaint. Because all rules was lies on the hand of the bounty manager in the first place.