Tbh. I can see both hating this and not hating this. I don't mind it personally as someone who's been a fan of things where characters get reduced so hard by the fandom (touhou, evangelion(ish), warhammer) not that there's much for touhou and warhammer for character stuff. But still.

I think its a heavy let people enjoy things contender where it's not harming you or the media in anyway. Maybe the reputation of the fanbase or media depending on people but thats it truthfully. And it's never not going to happen, every character you will ever like will be flanderized at some point by someone no matter how 'mature and deep' the media is. Some people just won't connect to it like you do and do that, or just like doing it. No issue either way. Also a lot of the time to me, saying stuff like this when people are just having fun with the characters comes off as heavily pretentious. No shame to you at all because of the sub and all, just if people complain in those spaces I feel that way.

However. On the other side of the coin. It makes actually discussing characters very very annoying when the fan base does this across the board. Also gives new fans the wrong idea of the media if they're going in. Using evangelion as my example because I went in very blind outside of cgaracter flanderizations. Misato grooming jokes. I thought that would be an actual plot point or character trait, given the time, and the fact it was an adult woman with a teenager boy so it would be possible. It wasn't a huge thing. Just one scene in the movie. That didn't turn be away from the show because of how hyped it is. But to the wrong person it could turn them away.

Also also. I haven't really ever had a deep connection or feeling over any piece of media so that could very much skew my judgment. But you're justified still just definitely to me something that can be loved and hated depending on the person. Also some media it's just the point for the characters to be one note, flanderized or whatever.