“Creative differences”—it happens to every band.
Absolutely... well perhaps not every band, but it is something that does happen quite a bit and I think it's reasonable to conclude that's what is happening her.
True, the “stay together happily forever” groups don’t make the news very much, sometimes bands break up because people are OK with the music but have come to hate each others’ guts, and sometimes bands do go through patches of “creative differences” and then work things out.
I like the way that the series has slowly built up the idea that Mitsuki really does have something special as a musician (the video in the park going viral, the jaded music producer pricking up his ears at her music, etc.), but it will take some doing for her to find her niche.