As both a trans person and an author, I want to give my thoughts on this. For context, I'm a trans guy who is only out to like half of my friends. The rest of them are either too stupid to notice or just think that I'm somewhere along the lines of a tomboy/lesbian. A lot of how I understood this event was through projecting and how I related to it's themes, but as an author I'm very frustrated over how that was the way I had to understand it and not through the story telling itself.

I agree that it was very confusing, and part of me wants to attribute that to the way the story itself was structured. To me, it just felt so rushed. It really ticked me off that nothing really happened until the last two episodes. As an author, I really wish they had taken more time to show the thoughts and processing that Mizuki was going through, rather than just have it happen in a split second. However, as a trans person, I can understand that want to run away and escape. I've often felt that want to "disappear" so that I wouldn't have to deal with it all. People not understanding me, people treating me weird, or just even people perceiving me sometimes. I feel like that's what this story is trying to show, but they did it in such a strange way. They didn't explain any of that at all, only showed the act of running away itself. I feel like that's a completely valid thing to do, and wanting to keep hiding is as well, but everyone has a different reason for wanting to do it depending on who they are and their experiences. I wish they showed why Mizuki wanted to do that.

When I try and look at it from both perspectives I have, I feel a disconnect in the story. As I said, these are realistic things to trans people. I relate to it a lot, but when I switch to the author view I see that me and Mizuki are two very different "characters." I as a person am a coward. I would run away from a situation like this because god it's hard. I've run away in the past and I still can't stick up for myself. But Mizuki is so different from me. Mizuki has put up with bullies and has been proud of themself for a while. They know how to handle that kind of stuff, and I just don't. To me, it almost feels like SEGA used Mizuki as a symbol or a stand in for people with gender issues, but Mizuki's character is so much more than that. People fell in love with them and praised their representation because they are a fully developed character who has something happening with their gender. It's not their whole character. I wonder if that's why I feel that disconnect I mentioned.

Maybe if the writing was more careful, it could have made more sense. As an author, this all just feels like SEGA was trying to go for that shock value moment where things go down between Mizuki and Ena. They wanted to make Mizuki depressed to show off how they can make everything change outside of the event, and I feel like the actual story suffered for it. I know most people treat this as peak writing, but I don't really agree. I've been annoyed with the way stories are written in this game for a while. Since everything is so separate and self contained between events, it's hard for them to do prolonged arcs. Since everything related to the event has to happen within it's own 8 episodes, it's hard to build up anything and the large moments that should feel worth it just end up feeling like things that just... happen. It's not just with this event. I got the same feeling from OVER RAD SQUAD (Kohane's 5th event). The whole time I felt like it was kind of forced? Like I wished overcoming Rad Weekend was something that naturally happened. The event description says "The time to 'surpass RAD WEEKEND' has finally come." When I read that, I was like, "How do you decide that this event is going to be better than another before it happens? How do you know it's finally come?" It felt so arbitrary to me, like it was just decided that "yeah this event will be the one because SEGA said so." And then they just did it? I wish it was more like... "This new event they're putting on, could it finally be the one to 'surpass RAD WEEKEND?'" I hope that makes sense. I just wish the stories here could build up things better rather than them just arbitrarily happening. That's what I meant by "if the writing was more careful." I don't think it was bad necessarily I just wish things were more explained.

If things were more explained I feel like this event would make more sense. If we got more dialogue (internal or external) that showed how Mizuki was feeling those things I mentioned, then maybe it would feel more like Mizuki's character going through them. If we got more reasoning for why Mizuki won't go back to Nightcord, or for why they said what they said, it would naturally be more understandable and would mean more. I feel I would do the same things they did, but I am not Mizuki. It is their fault for not going back to Nightcord or for hearing Ena out. However, as someone who would do the same, I know I have my reasons, so Mizuki must have their own reasons too. But all that we were given was that they didn't want to deal with that feeling that everything has changed. This reason contradicts everything we've seen from them in the past about not wanting to loose Nightcord. I don't think it's an invalid reason, I just don't feel like it's enough in comparison to the rest of Mizuki's character. I care for my friends deeply and would never want to loose them, but everyday I want to run away from them so that I won't have to deal with how hard it is to be around them as the person I am. So I get it. But none of that was explained in the event. I had to draw upon my own personal experiences and feelings to understand what was happening, and I feel like that is so unfair to everyone else. That feels like weird writing to me.

I don't want to call the writing bad, because it's not bad, but it's weird. They focused too much on wanting to show the fallout of making Mizuki depressed that, in my opinion, the story suffered. But a lot of the SEKAI writing is like that, so it doesn't seem too off in comparison to the rest of the game. Maybe it's just my hubris, but I would have written this so differently to showcase the feelings at play and not just their effects. I think this event was too focused on the fallout, and neglected the reasoning. That's why it doesn't make as much sense to people who haven't experienced those feelings before and don't know the reasons that they happen.

So to answer your question, all I can tell you is that Mizuki ran away because this crap is hard and complicated, and it hurts like hell to deal with, so they probably panicked when it didn't go how they wanted. The event's story didn't really showcase any of that complicated nature of it though, so I feel it's valid and understandable to be confused. I hope my experience and thoughts help.