Hi, this is on EU, hardcore game mode in case that matters.
Someone on Season 21 - Trials of Tempests: Double Bounties mentioned that “You have to exit game and create a new one, and keep doing so until you get a game with the buff active.”
I just wanted to give feedback here that I tried that, I restarted bnet and had the text for the buff active. Then I started a public game. When we finished the bounties and collected we all got only 5 boxes. I looked at the buff icon and the text was missing again. So sometime during the grind, the buff disappeared. So even starting a game with the buff active is no guarantee.