Look it's not like I'm trying to be the bad guy here, but she just told this girl, who is supposedly her best friend and who she has been living with for a year, that she had a girlfriend this whole time and she never said a word because she was scared she would treat her like a creep. That's just mean.
It doesn't help that Fukami has essentially been strung along by someone who she thought was single. If you see this from her perspective, she was just earnestly trying to get Kase's attention (in a socially awkward jock way, but still) and suddenly how she acts in this arc feels a lot less annoying.
It's unfortunate that this nice, important moment for both of their characters gets overshadowed by the poor writing. Frankly, if I were Fukami I would be piiiissed. Not at Kase, of course, she is perfect and amazing and can never do any wrong. Probably at Yamada though, she just has a very punchable face.