It's not quite him making a mistake, although that probably is the trigger for many of his smaller memory losses and in a basic sense what is going on. However that doesnt fully explain it. Even if he is blameless, it can trigger if he feels like he has failed or has become imperfect. It seems like there are two kinds of memory loss, the small details he loses because they're unimportant and he can "throw them away", and the large scale memory losses which seem to come about due to a major indecent or revelation. It seems like we are being led to believe that Souichi legitimately forgot the StL game because it was such an unimportant memory due to the lame gamble Baku proposed and how easily Leader beat him. I think at some point Souichi was able to rationalize the memory loss as a coping mechanism to relieve some of the pressure he was putting on himself, but our current Hal is tired of forgetting everything and thinks that getting all of his memories back will help him truly be perfect. The way Hal/Hachina has been talking, we know deep down he knew he was going to have to become Souichi again eventually, so in the back of his head he probably knows whats going on.
I don't know if Baku knows what specifically causes Hal's memory loss. But since he knows about it, and he knows it has happened multiple times from his experiences in the bookstore, he can probably glean that there is likely some kind of trigger for it. Perhaps a traumatic experience, or perhaps he can glean enough from knowing Hal that it is related to his insane sense of duty and responsibility. I think this is, ultimately, why Baku confesses to Yuukiide that he "killed" Hal. Baku is perceptive; he might have realized the reason Hal went to that gamble was to bet his life as Hal, but he lost and got blasted with a paintball to the temple to trick everyone (including him) into thinking he had lost the Russian Roulette game. This causes him to reset to Souichi, losing the "Hal" Baku knew. https://mangadex.org/chapter/713885/16 Initially I thought Hal in this scene was just extremely disoriented and shell-shocked and easy for Kakerou to imprint upon, but I think he straight up loses his memory again right here because he loses to Fukurou (and does not fulfill his desire to meet Baku again as "The Real Hal"). He was just coincidentally right next to Eba's place with book in hand, so he could immediately get his memories back (sans any memory of being Hal, Baku's friend).
This triggering of this particular incident is interesting actually, and a good example of what triggers the memory loss. Hal did the best thing he could do in the game up to this point, making the best hands for Baku. But with no knowledge of the rest of the game, the first two rounds were pure luck, and the third was the bottom room testing theories. Regardless, Hal's brain was "punished" twice to the point where his psyche feels like he must reset ; the brain was tricked into thinking it was being killed twice, and after both instances he actually BELIEVED he had been dismembered. Dark Souichi specifically points out the physical injuries and makes no reference to the poker game. So it tried to reset both times (Team Duwang points out the subtle reveal of it on the first loss). His brain basically went "okay, you're dead. your body has been dismembered and impaled. we need to start over". The pain is so extreme he almost forgets about the entire gamble, and he's able to fight back against it by realizing that resetting now will actually lose the overarching gamble.
I wonder what kind of personality we'll get from Souichi going forward. Re-reading his first major appearance in the manga (Hangman) he takes a very playful and rather whimsical stance which grows more serious as we work our way through the tower game and the gambles he makes with the police to consolidate Kakerou's power. From the Hangman game onwards, Souichi is extremely serious and driven. Hal did not really act like either of these Souichis at all, so I wonder what kind of person we'll get. Probably some combination, based on the aliens joke.
A couple things to note, a few interesting or fun things and a couple things I passed off as just Souichi being weird or quirky, but is foreshadowing that the author intentionally obfuscated so we wouldn't catch on.
https://mangadex.org/chapter/204569/12 Leader "meets" Baku for the first StL game (remember when the scanlation group called it "the mansion pass" game?). We know later that this isn't the first time they've met, but it is presented as such. It wasn't necessarily a lie; Souichi probably had forgotten his time as "Hal".
https://mangadex.org/chapter/204634/17 and https://mangadex.org/chapter/204634/18 A good example of probably the biggest case, Leader professed to not know Baku despite playing StL against him, the highest level of Kakerou gamble there is. He jumps back and forth between remembering or not remembering details here, as well as collecting or not collecting on Baku's life.
https://mangadex.org/chapter/204596/16 as catx3 noted, Souichi says he doesn't recall telling lies in this arc, so he probably legitimately forgot everything about Baku.
https://mangadex.org/chapter/204601/10 and https://mangadex.org/chapter/204601/11 This part is interesting. Souichi debates whether or not to call Baku's bluff that he will have Marco or Kyara come to the tunnel to stop the collection. He says he has never misjudged a situation like this, and makes the correct call here with Kyara lurking about, but how would he know? Any mistakes in judgement he's made were washed away. He has a few chances in this arc to get rid of Baku but doesn't do it.
https://mangadex.org/chapter/637839/19 Duwang notes a few examples of Leader's memory issues, particularly the part in Chapter 100 I mentioned earlier. This is Toshio playing games with us; Souichi in this scene talking about the benefits of losing his memory also just toys with the Judge about forgetting he isnt supposed to hit his golf balls. Since he knows the exact number, he's probably just fucking with him, which casts doubt in the readers mind that he has memory problems at all. A good red herring.
https://mangadex.org/chapter/204663/13 Leader remembers being told that Usogui (the guy he met in the mine) had challenged StL, but when Touya corrects him with the name Yuukiide he just accepts it. It was likely that Souichi understood this meant the alibi had shifted, but its also plausible that he believes that he just legitimately forgot or misremembered that detail.
https://mangadex.org/chapter/627724/2 Dark Souichi. At first I thought this was just a cool sinister effect of the memory loss, which it is, but its supposed to very directly represent the moment he gets his memories reset https://mangadex.org/chapter/869518/14
https://i.imgur.com/98Schhd.png Baku's friend Hal realizes that if Baku challenges StL, he will directly fight against his Dad.
