[Mithadan]

It is possible that Tolkien viewed the Mithrellas tale as "legendary" though perhaps likely. Or this question may turn on Tolkien's intent when he used the word "union." The three "unions" appear to have extended beyond mortality (not sure how to phrase this simply). Luthien took the fate of Men. Tuor was deemed to be of Elven-kind (or, alternatively, they are both sleeping on one of the Shadowy Islands), and if so this was only done with the approval of the Valar. Arwen also assumed the fate of Men. They were not parted by death.

Mithrellas was not bound by the fate of Men. She may actually have left her "spouse." So they were separated by the Man's death, unlike the others.