God, the author really has succeeded in making Yuni just the absolute worst. Hmmm, my lover is clearly distraught, and is shaking... guess I'll just leave her with this woman she clearly doesn't want to be around ¯_(ツ)_/¯.
Also, is that Jesus imagery for Fuuko here https://dynasty-scans.com/chapters/my_girlfriends_not_here_today_ch31#15 ? Wild.
Yuni is an asshole that sees Fuuko as her savior. I think she was overwhelmed by the idea that someone like Fuuko could be in a position like that. Most of the imagery we see of Fuuko from Yuni's mind is of a beautiful woman that feels almost like an angel. Distorting that idea of Fuuko with how she looked in that moment probably caused Yuni to bail.
As a selfish and self centered person she seeks comfort rather than providing any comfort herself. On top of that she acts very distant when a person breaks the perception she has of them. Yuni felt like Nanase was pushing her away so she cheated. Now that Nanase is trying to be a better gf Yuni's views of her just being a gf that doesn't care about her is ruined, which leads to her growing distant from Nanase. I feel like the same thing happened here. Her belief of what Fuuko should act like is different from how she was acting here, which lead to Yuni pushing her away.
Her being self centered was also showcased. One of the main things that pushed her and Nanase apart was the idea that Nanase spends her time with others. That they get to see sides of her that Yuni doesn't. I think with this new woman it might be the same case. Yuni at the end might be missed that it's someone else that can make Fuuko break down so easily. That it's someone else that was her first partner. It takes away from Yuni's idea of Fuuko being her savior, and in Yuni's mind makes her just another sex partner. Even worse a sex partner that isn't able to see sides of Fuuko that the new woman was able to see. Similar to how she views Nanase and how she interacts with her teammates.