I completely agree that the rainbow lucky block should respond in all modes, as it means there will be good loot at the center of the map throughout the game, rather than just the start, which incentivizes players to go there and results in confrontations. However, keeping the randomness of the rainbow lucky block is part of the fundamental theme of lucky islands and should not be taken away, although I think there should be ways that players are able to deal with campers nonetheless.
Maybe after 2 minutes (this amount of time should be balanced and adjusted) of there only being 2 players/teams left, the next player/team that opens the rainbow lucky block at middle instantly wins, and a message in chat would say that. This would still give an advantage to the player who played offensively to make it to middle, and because the rainbow lucky blocks are always in an open area, it’s hard to camp or hide from other players while being near them.
Compasses greatly reduce this issue since you can now find where the last player is, but it doesn’t make it any easier to make it to players that purposefully go to difficult to reach locations in the map (tunneling underground, sky-basing, waiting on the other side of a 1 block wide bridge, etc.)