For a woman to be able to remarry after her husband has gone missing (e.g. after a war with no body found), she must wait 4 years before she can remarry.

In the Hanafi school, if the woman has remarried and the first husband returns, the woman goes back to the first husband and the custody of the children she had with the second husband goes to the first husband.

Let us look at the original opinion of Imam Abu Hanifa in the book Al-Hidayah written by Burhan al-Din al-Marghinani, in chapter 124 ("The wife of the missing person"), pages 379-380:

Abu Hanifa said: "The judge should not cause a separation between him and his wife."

But if she cannot remarry, the scenario where the husband returns and finds his wife remarried simply does not exist.

The reason this happened is that later Hanafi scholars found Abu Hanifa's opinion — that the woman cannot remarry if her husband is missing — too harsh.

So they adopted Imam Malik's opinion that she may remarry after 4 years.

This opinion is found in Imam Malik's book Al-Muwatta (Book 29, number 29.19.52) in the section "Waiting period of women whose husbands are missing":

Yahya related to me from Malik, from Yahya ibn Said, from Said ibn al-Musayyab, that Umar ibn al-Khattab said: "A woman who loses her husband and does not know where he is must wait four years, then observe a waiting period (idda) of four months, after which she is free to remarry."

Malik said: "If she remarries after completing her waiting period, whether the marriage was consummated or not, her first husband has no right over her."

Malik said: "This is what is done among us, and if her husband finds her before she remarries, he has more right over her."

But the text above says that the man does not get his wife back if she has remarried.

By adopting the 4-year rule that was not foreseen in their school of jurisprudence, and wanting to remain consistent with the rest of their school to avoid contradiction, they had no choice but to apply this rule that the missing husband reclaims his wife, as well as the children the wife had with the second husband.

When a school's logic overrides the text, it is the school one follows, not Islam.