>>103057082The actual answer is mostly nothing, all three just left for unrelated but understandable reasons.
Sana learned that she wasn't a good fit for VTubing, and so left early, making her the closest thing to a "hiring mistake" of the group. Mumei left because she was literally losing her voice, and Fauna left because she wanted to get married and not constantly travel to Japan apparently.
There were certainly some coherency issues early on, as well as living in Myth's shadow and being clearly thrown together quickly to capitalize on Myth hype, but over time those issues have mostly been smoothed out. So in the end, it was just circumstance that caused them to happen to be the first gen with three graduations.
