Leader either knew he'd lose his memory, or he lost it before the death. I'm positive this is the case due to the "Referee Yakou..." line out of him in the previous chapter. When dark Leader showed up in
Chapter 454, he said Souichi went through 138 memory losses, including the little ones. He's had a couple more since then, between forgetting the agonies and his sight. By now, even if the monthly ones are truly uncontrollable by him, he's still definitely aware of the trigger for it, given that the monthly loss happened at least 40-50 times (It's at least the year 2006, rather 2007 now, as we know from the Labyrinth arc).
If he knew it was coming, he could prime Yakou for it. Yakou was an attendant of Leader long ago, so he is fully aware of the process of bringing Leader up to date discretely. The real question, I think, is whether or not this memory loss was intentional by Leader, or something that he's now forced to work around. Provided Yakou does bring him up to speed (or Baku does it cause he doesn't want to beat an amnesiac Leader, who knows), this isn't necessarily a disadvantage for Leader. Lalo lost the Ban and Air Poker because of the wedge Baku drove into him that "Usogui will do anything to win", and Leader has similar concerns about Baku given his suspicions based on the dreams, and his time as Hal. If he forgets about Baku, he's in a case where he has a marked advantage of 5 safe checks to the opponents 3 checks. In this perspective, Leader could have opted to lose his memories intentionally alongside the near-death to completely kill his opponent.
I'm probably missing something important with these speculations, but I'm confident that Yakou (or Baku) is going to start talking about the match in detail to remind Leader.
Edit: I misread the years on the charts, the latest year shown was 2006 and not 2008, credit to
@RapidLord for making me go back and double-check.