Is that really a thing? My father is japanese and I grew up in Brazil. I don't think my lack of fluency in japanese was due to any fear from my parents - my parents spoke japanese at home and even had me attend nihongakkou (japanese classes). But it's very hard to develop real fluency without a community. My cousins had always been more fluent due to being involved in a karaoke community - something my family wasn't. My mother was born in Brazil too, and spoke japanese fluently as a child because she was raised in a isolated japanese farming community where many people couldn't speak portuguese.
Even now, with my kids, it's been far easier for them to pick up mandarin (my wife's mother tongue), since until a few months ago, their grandparents lived with us and spoke mandarin exclusively, and since the community where we lived was predominantly chinese (read: there are chinese supermarkets, there are chinese books in the library, and most classmates in their school are 1st/2nd-gen chinese). Compared to that, it's been a lot harder to get them to learn portuguese, since I'm literally the only person around them who speaks it.