I think you are over-thinking the whole thing way too much. As long as it's believable than it's fine. Does the book focus on their race in someway that highlights it? Are you overemphasizing race versus say, finding food in apocalyptic Australia? People talk the way they talk. It ads character. I don't think anybody is going to chastise you for it, and even if they do, it's such a silly thing that it does nothing except call more attention to your story, and then more people read it. I'm not saying you should intentionally make it obnoxious, but people will shout and scream about anything nowadays. Just write the story that you need to write, and don't worry about what people "might" think.
I don't think it's about what people might think, or about people shouting and screaming at you.
The OP specifies a concern for taking voices and stories from Japanese people. That's a real concern. It's not a"don't worry about people being offended" thing. It's a concern that comes from empathy for other human beings, and the worry that speaking on their behalf, in a sense, may obscure their own voices and stories.
Sometimes people worry about "offending" people because they genuinely don't want to cause harm, and framing it another way, I think, doesn't help with the conversation.
I don't know the answer myself. I struggle with this a lot. I still put people of other races or skin colors or ethnicities in my stories, because I'm white, and the alternative is to only write white people, which I truly believe is more harmful in the end . . . but sometimes, if I'm honest, I wonder if the only way not to do harm is to just stop writing. The world has enough white American voices, doesn't it . . .
But, I still don't know. Part of me thinks that's not right, either, to just stop altogether, so I keep writing anyway . . .
All this to say--I don't really know how to answer your question, OP--but, I get a bit frustrated with the interpretation of the question "is this harmful?" as "will this make people mad at me?" . . . because it's a real question, this "is it harmful" thing, and converting it to the other question lessens the meaning behind it, and doesn't help it get answered . . .
Not to...go off on a tangent. Except I sort of did? Uh. Hm. Well. I'm going to shut up now at any rate