I read an interesting book by a journalist who went to visit the Mizorams. He ended up being pretty convinced. They had old, corrupted heritages that resembles Jewish themes very much. They referred to themselves as children of Minmasse. They mourn what is to them the worst event in their history, when their last Sefer Torah was lost.
The Gemara says that if a woman gets Kidduushin from a goy she doesn’t have to consider the possibility of him being from the lost tribes. From here it seem that they lost their Jewish identity. But there are many groups. Yirmiyahu hanavi brought back some, B’nei Moshe were taken to a remote location, others became lost, and others yet are secluded.