yes, they have the right to extend but forcing participants and threatening them that their stakes will be forfeited is a huge red flag.
Have they threatened bounty hunters already? Can you give a link to a post where they said they will remove stakes if a hunter refuses to continue promoting.
Original bounty period had increased from 4 weeks to 16. That is a lot. I remember they had a discussion about continuing bounty for more 4 weeks. And now they quickly decided to add more 8 weeks. Seems suspicious.
They must understand that many bounty hunters do not like a very long campaign period especially those in the signature, they joined the campaign because they thought it will only last 4 months but now 16 months and if you leave no stakes , imagine from 4 weeks to 16 weeks, they must respect bounty hunters now many cast suspicion on their campaign, I have this feeling that they do not want to give bounty hunters their rewards.