(nosepinch) Guys. Jesus. Chill.
I am pretty certain "Did she bring a girl home??" is just because the lights were off and she heard the panic in Mitsuki's voice and couldn't in that moment think of an innocent reason for it. It's not anything as deep as Aya being insecure or having any real reason to doubt Mitsuki. It's not a "bait and switch" and it's not intended to make you think they're broken up. It is a momentary misunderstanding that explicitly shows that they're still together, because Aya wouldn't have rushed in angry if they weren't still a couple. Sometimes a situation just seems fishy and your brain jumps to the worst possible explanation and oh look it's almost like I am simultaneously talking about both Aya and this comment section.
I would argue that if your mind can't come up with an innocent reason for something then that probably stems from an insecurity. If not, how would you define insecurity?
Coming home early to your house in darkness with only a light from your bedroom and your partner yelling out shocked that your home early while shuffling and panicked? No, its not insecure for the random thought to come up and even less so if this happens as part of a fictional comedic chapter in a story.
I really don't think the author wants your takeaway to be that she has an insecurity that needs solving. I think the takeaway is "who is her roommate", "oh it's her GF," "hey they live together" and "isn't it funny that she burst in" and "OMG look at their new designs" and then "awe she was hiding a gift, how cute that they still love each other and music."
First of all, there are a ton of other explanations for that scenario, like preparing a surprise and not wanting it to be spoiled, or making a mess and wanting to clean it up before your partner comes home, etc. Immediately thinking of cheating points to either a society in which cheating is so common that that's the first that comes to mind (which technically does apply to Japan) or an insecurity. I certainly was not expecting Aya to think that, my mind didn't immediately go to cheating.
Second, I don't think either of us can say what the author was thinking, but also, I don't really care. I believe in "Death of the Author" and my reading of Aya and Mitsuki is such that an insecurity like that is not unreasonable.
Third of all, I find it weird that you're defending this SrNevick as I'm generally a fan of your comments and takes. I mostly responded here against the notion that "it's not that deep". I hate that take as far as fiction goes. It may not be that deep, but it is as deep as the reader allows it to be. I don't like the notion that because something is comedic or fluffy that it's worth less time spent analyzing than an intense toxic drama. And if this reads as just a fluffy light chapter to you, then that's great, I wouldn't argue against that, but that's not my reading. Both can exist because they depend on the reader not the chapter.