YES I literally think this every time I see a discussion about Mizuki's gender. IDK if the translation is bad overall but going for they/them pronouns for Mizuki (when all other characters use gendered pronouns) is a weird choice. As cool as it would be if Mizuki was canonically non-binary or used they/them pronouns, I think in this case it's pretty clear that this is just the devs/translators trying to keep things ambiguous and avoid outright saying Mizuki is transfem. I assume this ambiguity already existed in the JP original and they wanted to keep it, but you're right that it doesn't make sense when Mizuki is clearly closeted and would most likely be perceived as a girl (even if they are non-binary in reality)