I'll explain a few things about Ave Mujica from the last page as someone who follows the franchise that may shed some light to why episode 13 of MyGO is the way it is.
So when Ave Mujica was first unveiled, only the characters' stage names were given. They were pretty much an unknown to the community. Then the MyGO anime started to air and people noticed some of the characters shown looked like members from Ave Mujica, and that's when it was picked up on that they would be part of this anime too.
So people were able to piece out that:
Oblivionis = Sakiko (Keyboard)
Doloris = Uika (Vocal and Guitar)
Mortis = Mutsumi (Guitar)
Timoris = Umiri (Bass)
Amoris = Nyamu (Drums)
Now here's the thing, throughout the time the anime was airing, the Ave Mujica VAs only announced that they were voicing their characters as the character and not the stage name even though it was obvious to the fans who was who. So you get a Superman/Clark Kent situation.
Now episode 13 comes along Ave Mujica has suddenly formed, each character says their stage name, and now the VAs can formally announce which member they are despite it being no surprise. So in the end it was a big marketing thing and a setup for the next season.
I'll say that in the game, there's a retelling of the MyGO anime story and it ends where episode 12 ends. So episode 13 really was meant as a bonus.
Robazoid posted... I mentioned earlier that Rui from Morfonica was my least favorite BanG Dream franchise character. That position has now been taken by Rana. Ugh, she's so lame. She's just a pale imitation of Hina (instantly good at everything, finds other people interesting for odd reasons), except Rana added nothing to her group's dynamic.
So your reason is valid, but that's Raana's thing to her character. She's like a "stray cat" that does whatever she pleases and is completely detached to all the drama that the other 4 go through. She's an outsider that barged her way into the group. Considering her flightiness, I would say they were trying to not make her too much like Tae. They play the same instrument after all.
As for Mutsumi and Sakiko, there's a theory floating around between those two, but that's for next season to see if anything comes to light.