After World War II the children which were placed in Catholic families often found no Jewish families to take care for them or to bring them up in Jewish traditions. Most of them even did not have the opportunity to learn Hebrew and got estranged from Jewish traditions. They married and got children who started to wonder about their ancestors. Several of them also wanted to go back to the Jewish culture and found stronger bounds with Jewish religion, whilst others wanted to keep their faith in the Messiah, but did not want to be counted under the trinitarian Christians.
Looking at the results of two surveys (one from 2013 and one from 2017) which mainly looked at one group of "Messianic Jews" instead of looking at all sorts of millennial Jews.