Staff, thank you very much for translating the chapter.
I am guessing Umi made his move and Mizuhara told him to shove it. She did not seem happy after talking to him, and even though Kazuya was beginning to like the party, she asked him to join her in leaving.
For context, those who don't recall, during the movie arc, Umi promised to help Mizuhara if she went out with him. He then asked her out, she declined, and when he asked if she liked Kazuya, she said, "I don't like him but I don't NOT like him." At this party, Umi talked to Kaz for a while and sent him back to Mizuhara saying those exact same words to her through Kaz as the unsuspecting messenger.
Some of us, me included, thought this was a smooth and calculated maneuver to take Mizuhara for himself, others thought he was being a wingman for Kazuya and helping him get closer to Mizuhara. Considering that 1. she looked thoughtful but upset after Umi said some stuff to her that was not explicitly revealed and 2. she asked Kazuya to join her in leaving right after talking to Umi, it seems I was not mistaken about Umi's intentions. Mizuhara left his party right after he talked to her, and she left it with Kaz. Pretty easy at this point to read between the lines both as what Umi wanted and whom Mizuhara herself wants. The relationship talk with Kazuya, given that she knows at this point that Kazuya loves her, is not an accident on her part. That's not how she works. On Kazuya's part, he's explicitly ready to make Mizuhara his girlfriend, which is pretty much what she wants at this point herself.
Umi helped them get together alright, but only after Mizuhara told him to take a hike first. Poor actor boy didn't get what he wanted for once.