Hmm. Is this informed at all by the corner case of Sacred Grove on the River's Gift?
Actually, now I think about it, Sacred Grove + River's Gift is a bit peculiar in a lot of ways. I'm guessing players have to choose whether or not they are going to receive the Boon when Sacred Grove gets played, but then each player's choice must be remembered until clean-up?
Then, "(Keep this until Clean-up)" is part of the main on-Receive text, not below a line. So... um... every player who chooses to receive the Boon must keep the card? I sense a custody battle.
But, more pertinently, The River's Gift is presumably discarded at the end of the current player's turn, not retained until Clean-up of the last player who chose to receive it. That would tend to suggest that on-Clean-up effects happen the next time any player cleans up?
(Or maybe the person who played Sacred Grove keeps The River's Gift, and everyone else Loses Track of it. But the Lose Track rule applies to Cards, and The River's Gift isn't a Card...)