Because for some reason Kazuya decided to have that date with Mami in Chapter 263. When he is already under the same roof as Chizuru and Chizuru supposedly already investigating her feelings.
Mami was kind enough to not out Chizuru for her rental agency, which she totally could do at anytime, but it's not like the agency bullshit ever really mattered. Just look how the parents reacted to the reveal.
Therefore, Mami was clear on that meeting, believing Kazuya won't pull her leg again. But Kazuya is not telling her about the investigation part, neither the cohabitation part. On top of that, Chizuru is making the situation worse as she told Mami that no, they are not dating, so the kissing shouldn't be a big deal.
Chizuru threw back the ball to Mami's court at the end of Paradise. Shitty resolution, I know. Mami is the one who should investigate. Mami decides to do so, and the #1 threat assures her they are not dating, while Kazuya is not saying anything on the matter because he knows Mami wouldn't be pleased to hear his status after how the previous shitshow ended. But went to a date anyways, which totally could have made Chizuru doubt him again, just saying.
So more secrets, more lies to the side characters, rivals. They learned nothing from Paradise. Somehow Chizuru can't face the girls honestly and tell them that the initial rental relationship has evolved. Of course she can't, that way all the accusations she received through the manga will be proven true. Kazuya still waits for her indecisive ass, but to do so requires time stalling lies again, again and again. Just like the manga, these web of lies are neverending or it seems.