Bug: saga doesn't go in graveyard as soon as it should

[rex4]

As you can see from my post an hour ago, I'm currently solving puzzles so it's likely I will find more bugs too:
Puzzle possibility storm Dominaria #07 is impossible to solve because part of the solution is "sagas are sacrificed the moment their last ability has left the stack, meaning the Eldest Reborn is already in the graveyard when Rona enters the battlefield and is thus a valid target", in forge whatever I do the saga is still on the battlefield when Rona ability happen.