So you think they raised Marco bounty after MF and then lowered it when he lost against BB ?
No, they never raised his bounty in first place. The Goroseis had expectations for him and the rest of the Whitbeard pirates, thinking that they
might be able to take down Blackbeard or some of his crew members, but they failed miserably. Marco never officially took over the crew, they fought against Blackbeard as friends, as equal sons of Whitbeard who were driven by anger and the grief of their loss. After that catastrophic loss Marco retired piracy, for good. Marco never became the new leader or captain of the Wh pirates, so he never got any bounty increase in first place. If Marco was the official captain of the Wh pirates, even for the time being, then yes, he would have gotten a bounty update, but as we know he never did.