person from country A moves to country B. Both countries speak English. A few years later A moves home. People in country A now hear a country B accent when this person talks.
When you're from country A, but move to country B, you don't feel at home in country B. But when you return to country A, you also don't feel entirely home in country A. You have a slightly different accent, have had different experiences, likely developed different cultural mannerisms, habits, etc. etc. .... You're perpetually stuck between cultures, hence Third Culture Kid(TCK). Especially true of children who move at a young age. So much so that, if they read a book about TCKs, they soon realise that what they thought were unique personality quirks, strengths or character flaws. are anything but unique among fellow TCKs. Also true for adults but to a lesser degree. Also true for people who moved larger distances within a country, changed schools a lot, etc.
A bit like returning from a holiday and finding things have subtly changed, if you stay away long enough you'll likely never feel entirely at home again. The country you knew no longer exists, you're no longer the same, you notice it's not the same, and people notice you're not the same.
Of course, in some ways nostalgia is similar. Nostalgia is homesickness for a place that existed in the past. Like waking up one morning and discovering someone rearranged all the cutlery, it can make you feel perpetually discombobulated.
Unfortunately/fortunately, there's no way to stop time, and Make the World Like it was Again. Not that populists and advertisers won't try to sell you on that lie.