I'm probably going to get hated for this, but...
As much as I appreciate Mitsuki's character growth to becoming assertive and confident (and as much as Aya clearly appreciated that little move as well), Mitsuki is still showing facets of jealousy, control, and lack of trust. In real life, these things are huge red flags.
If someone had come up to Mitsuki and asked her about Western music, she would have gone into full "geek mode" and chatted them up non-stop; if said person had done that under the pretense of flirting with her, Mitsuki would not have noticed; she would have been ass-deep in gushing about Eric Clapton and George Harrison 0.38 seconds upon hearing the song playing on his phone. So, for her assume Aya couldn't deal with this shows a rather meaningful lack of trust, and maybe even a bit of projection.
And, considering the meaning of the song... Patti Boyd made her choice, and George Harrison respected it. Mitsuki would know that.
So yeah... I love this comic, I've loved it since the moment I first laid eyes on it, but I can't say I like this particular entry, and I hope it isn't the start of a dark path for Mitsuki.
Not sure I agree with that. If you ask me, rather than Mistuki not trusting Aya, it read more like "is he really trying to hit on her? Oh right, that reminds me, we haven't actually addressed if we were dating or not. Might as well ask." And then she did just that and asked at that moment because what teenager has never had any impulsive acts.
If you notice, not once did Mitsuki tell Aya or the other guy not to talk. All she did was ask and left them to it thinking she could get back to her conversation with the former bully. She probably wasn't even thinking (at least not fully) about how that question would affect the other guy and Aya's music discussion.