>>107854635NTA but at this point who the fuck knows or even cares anymore
Her story is that she was supposed to join Hololive (in a tangent where she praised Niji) but she got talked out of it by Mowtendoo. (How the hell did he even know she was joining?) and the timing lines up in a way where she was supposed to graduate a week before the others. However, she announced coming back directly after the black stream. Like a few hours later after everyone realized just how much of a colossal fuck up it was.
The problem is that Grimmi is known for being a frustrating fence sitter desperate to please all sides no matter the situation and regardless of her personal feelings. In literally any drama that happens, even if she's not involved in any way, she will come out of nowhere and start trying to "deescalate" by appealing to both sides. On top of this, she's both incredibly impulsive AND extremely gullible, so it's perfectly in line with her character to see that Niji's fucking exploding and get convinced by Mow that Holo's the same way and quit, but it's ALSO in line with her character to join Niji, see that everything's fucking exploding, leave on her own, and THEN overcompensate by trying to praise Niji and pretend she was joining the one that wouldn't care or call her out if she did or didn't join.
The only correct way to interpret this situation is that Grimmi has a fucking brain tumor and to not bother with it.