See what I think fumi is angry about is still having sakura's shadow hanging over her and shionbu when it comes to shinobu he still thinks about her even with fumi right in front of him. She asked him to look only at her and even if it's just for the script to get to know her he hasn't done that. That's why she is acting out. What's interesting is you are right we don't know what fumi is to shinobu it is safe to say sakura and fuckboi are yes in a relationship how romantic that relationship is is the question not are they but with shionbu we still don't know what fumi is to him normally it's sakura whose thoughts on a matter that are hidden from us. The answer to the question of what is fumi to shionbu will be the answer to the end of the manga I thought we would at least have an idea as to what that is by now but no we will not know until the very end.
I mean the two relations are not in their best moments and i think they'll both couples path ways.
I don't think that the relation between Fuckboi and Sakura is not good in romantic aspect and for Shinobu and Fumi we were left in a ambiguos aspects about Shinobu's feelings about her.
So i think that will pontetially lead for both relations being over in the next chapter because of one circunstance both boys:didn't understand try to understand the girls side and we saw that in 134 and 135.
And about Sakura's toughts,one think that we know is that she values her story with Shinobu and with her own words she said he was the responsible for her to do her best,we had a parallel in this chapter that Shinobu say the same thing about Sakura.
And her relation with Fuckboi it looks like he didn't even try to understand Sakura's side and i don't that relation like she had with Shinobu.I could say is worst than her relation with Shinobu because Shinobu really supported her until the end.So i think Fuckboi potentially will lose Sakura for the same reason Shinobu potentially will lose Fumi:Not trying to understand her feelings.