Giving a reason for your sudden drastic change in relationship is the Least you can do, But the one thing we know about Hiragii is that She isnt sociable so...
It's bit of a plot point in the twins' larger character arc that their insular co-dependence growing up has left both with badly underdeveloped social skills, though in contrasting ways that play on their wider Sibling Yin-Yang themes.
Tsubaki is shy, timid and can just about go into panic attacks around strangers - but once the initial ice is broken actually very sociable and open, and seems to find it naturally easy to establish meaningful relationships with others. She's also something of a fascinated observer of people though normally too shy and timid to ask personal questions; she was positively rapturous when she had an explicit permission to do so during the sleepover with the gyaru pair.
Contrariwise, Hiiragi is willful and outspoken but also extremely reticient and "closed off" - by her own agonized admission she doesn't have the slightest idea what a friend even is never mind now how you'd go about making one. (Utsugi definitely had to do all the heavy lifting there, and Akutami-sensei still had to spell it out to her...) Compare how Tsubaki could lay out the whole mess to Yatsude literally on their first meeting while it took Hiiragi months of familiarity and hitting critical mass rock-bottom self-loathing depression to unburden herself at Akutami who was basically the closest thing she had to a confidant at the time. And even after she got the worst of her personal demons exorcised and accepted Utsugi's overtures for what they are she, tellingly, appears to have a very consistent "don't ask, don't tell" approach to personal matters much to the shorter girl's frustration - not that Utsugi herself was exactly volunteering such details mind you. (It's been rather implied the Manic Pixie Dream Girl has her own baggage.) One rather gets the impression Hiiragi just baseline isn't interested in other peoples' business - though she's an astute observer when she has a reason to pay attention, something of a shared trait of the twins - and accepts them, or doesn't, as-is.
'Course it hardly helps matters that by all indications by the end of middle school Hiiragi was basically a bitter, depressed, self-loathing wreck who just desperately needed to get away from her sister one way or another. Niceties were far and away the very last thing on her mind, all the more so as given she was just as co-dependent forcing the very much necessary break oughta been about the emotional equivalent of gnawing off a trapped limb if you pardon the gruesome analog. Note how for all the carefully studied poker face she's in her Dull Eyes of Anguish mode through that entire scene.
