I don't think it was cut, not yet at least. Maybe moved instead. They've re-ordered the sequence of events to flow a little more naturally in a visual medium, since the LN does a lot of retelling of events from another character's perspective.
Chapter 1 of the LN covered Masato's perspective throughout his day with Koumi, which was covered in chapter 2 of the manga, whereas Koumi's perspective that appeared in the same manga chapter wasn't covered until LN chapter 9. Chapter 4 of the LN covered Koumi and Masato's first meeting, which was covered in chapter 1 of the manga.
So some things have been re-ordered a bit to consolidate more focus and have the character perspective flow more naturally in each manga chapter. The trend has been to introduce the scene with each girl from Masato's perspective, then switch to the girl who keeps that perspective throughout the remainder of the manga chapter, and the reader gets to see the contrast between how Masato innocently perceives the scene and how it not-so-innocently impacts the girls.
Aika does indeed warn Masato about Seira in chapter 3 of the LN right after the part where Seira tells Masato that he can't see any other women at the boy's bar, but this warning is from Masato's perspective, whereas the latter half of manga chapter 4 is from Seira's. We would have had to change our perspective from Seira back to Masato in the manga, and I don't think that would have flowed quite as well.
What I think is that Chapter 5 of the manga will begin the following morning with Masato's perspective again and offer a flashback to Aika's warning from the night before. This would include the warning (that we as readers will know is already too late; but the premise was spoiled in Chapter 1 of the manga, so it's not like we didn't expect Seira to grow needy anyway) and allow the perspective to stay with Masato as he moves on to the next plot beat that same day and introduce his other part time job... tutoring the next girl.
