Apparently, Sora’s disappearance was not a coincidence at all.
I admit that this episode was somewhat confusing for the fact we don’t know why Sora abruptly disappeared. While I haven’t read the source material, apparently it covered the game of existence Othello/Reversi a lot better for the fact that the game was seen from Sora’s point of view. Not only that, Shiro was able to see the board in the light novel. Because of this, she would know how the game was going instead of being left in limo. Moreover, it wouldn’t give the audience the wrong idea that they won because “Blank” is invincible.
As for the technicalities of the game, apparently Sora loves to play games with high stakes. Therefore, he decided on a game of Othello, which the challengers put their existence on the line. Just like chess, Othello is one of those games that you can trick your opponent and eventually change the tide completely. The corners and edges are usually the most valuable places to win. Therefore, when it was Shiro’s turn to play, knowing her brother’s strategy and what she knows about the game, she only needs to put the remaining piece in an empty corner to instantly beat Clammy.
Strategies aside, I think one of the main reasons why Sora deliberately played a losing strategy is for the fact that he wanted Clammy to get rid of her distrust against him. As revealed in the last two episodes, she actually believed that Sora and Shiro were working with the Werebeasts. From this, Sora knows that if he wants help from the elf friend Clammy has, he basically has to win her trust. Therefore, he intentionally played a bad strategy so that she will have his memories temporarily before Shiro can come in and change the tide completely. While Clammy lost her soul for a few minutes, Sora allowed the participants to keep a copy of the memories they stolen in addition to getting back their own. In other words, nobody actually lost, except for Feel.
As a whole, while this episode could have been better, at least we know that regardless of the situation, Shiro still believe in her brother. This is why she decided to make a wining move in a chess match against Jibril at the last-minute. Aside from that, with Sora winning Clammy’s trust, it’s just a matter of time until they finally find out what kind of game the werewolf plays through Feel’s magic.