You aren’t imagining things. She indeed no longer “true Japanese.” I have met many such people who went abroad for a number of years and then after they return, they just don’t fit really well into Japanese society anymore. Most of them will tell you that themselves. One particular friend, Keiko, went to high school and college in California, then worked for a few years until she was 30. She returned to Japan and has lived there since. She’s 50ish now and tells me all the time about the conflicts she has with “the Japanese.”
For Japanese, the important period is high school and college. Spend that entire time overseas and culturally you are a foreigner forever. Younger kids who spend some time but come back for high school seem to not have that same effect.
Adults can go either way. I know Japanese who worked for several decades overseas but are still just as “Japanese” as the day they left, or who quickly returned to being “Japanese” once they moved back.
OTOH, I know Japanese adults who moved overseas and then never wanted to return.
Japanese society is intense and the group think is deeply embedded. For some reason, spending high school and college years outside of that society forever changes a person.
(Of course, there are individual exceptions, but this is one of the generalisms I’d bet the family farm on.)
This isn’t the case for many, or probably most societies, which really shows how insular Japan is.
It really depends on the group. Some groups are more inclusive, others are not. Japanese is an extreme case, and it’s all of those, ancestry, language and lifetime experience and bonding in the society.
That misses the point of the story, as shown above. There was never a question if the girl held a Japanese passport, but rather if she was culturally Japanese. And she obviously wasn’t because people were telling her that she wasn’t.
It’s a difficult situation for many of them because they wind up not being seen as Japanese by the Japanese, but also not being seen as French, American, or whichever country they have lived in. Many people I’ve talked to say it’s often quite lonely and frustrating.
I grew up Mormon and after having left the faith, find that visiting family is so different. I speak a different language now, have different values and don’t share that sense of being “an insider.”