not Aya, Mitsuki.
It was back in chapter 150. Mitsuki's dealing with going through bands and not finding a group that clicks with her, and then she stumbles into that girl band where they're all super focused on socmed presence. Mitsuki kinda stops talking about her struggles with Aya during this time and withdraws a bit, but comes home late in that chapter while Aya is stressing over Uni Work, and Mitsuki's just really out of it and distracted and gets super clingy out of nowhere and glomming onto Aya. Aya thinks "something must've happened" but she decides she'll ask about it later (because Mitsuki only just got home and Aya's in the middle of important homework stuff), and then Mitsuki just hugs her from behind and gets affectionate immediately.
The final bit of the 4th page has Aya going "wow she's being spacy and now she's being clingy....THAT MEANS SHE'S CHEATING" and it reads like a throwaway gag that's just Aya being real extra with her jumping to conclusions--and it's promptly never brought up or referenced again.
But with the show that Mitsuki is stressed over happening, and Aya surprising her by showing up and Mitsuki having a "crash 'n' burn" moment, Mitsuki pulls back and they start avoiding one another more and more. So people latched onto that "SHE'S CHEATING ON ME" and assumed it meant Blue Yuri / a breakup was in the works.
And that was only exacerbated by the introduction of Shu, and now Rinko.