That Our Lady died follows from Pius XII’s Munificentissimus Deus 14,17-18,20-22,28,35,40.
From the Apostolic Constitution of Pope Pius XII: Munificentissimus Deus – Defining the Dogma of the Assumption (November 1, 1950) at the Vatican.va website:
14. Christ’s faithful, through the teaching and the leadership of their pastors, have learned from the sacred books that the Virgin Mary, throughout the course of her earthly pilgrimage, led a life troubled by cares, hardships, and sorrows, and that, moreover, what the holy old man Simeon had foretold actually came to pass, that is, that a terribly sharp sword pierced her heart as she stood under the cross of her divine Son, our Redeemer. In the same way, it was not difficult for them to admit that the great Mother of God, like her only begotten Son, had actually passed from this life. But this in no way prevented them from believing and from professing openly that her sacred body had never been subject to the corruption of the tomb, and that the august tabernacle of the Divine Word had never been reduced to dust and ashes. Actually, enlightened by divine grace and moved by affection for her, God’s Mother and our own dearest Mother, they have contemplated in an ever clearer light the wonderful harmony and order of those privileges which the most provident God has lavished upon this loving associate of our Redeemer, privileges which reach such an exalted plane that, except for her, nothing created by God other than the human nature of Jesus Christ has ever reached this level.
17. In the liturgical books which deal with the feast either of the dormition or of the Assumption of the Blessed Virgin there are expressions that agree in testifying that, when the Virgin Mother of God passed from this earthly exile to heaven, what happened to her sacred body was, by the decree of divine Providence, in keeping with the dignity of the Mother of the Word Incarnate, and with the other privileges she had been accorded. Thus, to cite an illustrious example, this is set forth in that sacramentary which Adrian I, our predecessor of immortal memory, sent to the Emperor Charlemagne. These words are found in this volume: “Venerable to us, O Lord, is the festivity of this day on which the holy Mother of God suffered temporal death, but still could not be kept down by the bonds of death, who has begotten your Son our Lord incarnate from herself.”(11)
18. What is here indicated in that sobriety characteristic of the Roman liturgy is presented more clearly and completely in other ancient liturgical books. To take one as an example, the Gallican sacramentary designates this privilege of Mary’s as “an ineffable mystery all the more worthy of praise as the Virgin’s Assumption is something unique among men.” And, in the Byzantine liturgy, not only is the Virgin Mary’s bodily Assumption connected time and time again with the dignity of the Mother of God, but also with the other privileges, and in particular with the virginal motherhood granted her by a singular decree of God’s Providence. “God, the King of the universe, has granted you favors that surpass nature. As he kept you a virgin in childbirth, thus he has kept your body incorrupt in the tomb and has glorified it by his divine act of transferring it from the tomb.”(12)
20. However, since the liturgy of the Church does not engender the Catholic faith, but rather springs from it, in such a way that the practices of the sacred worship proceed from the faith as the fruit comes from the tree, it follows that the holy Fathers and the great Doctors, in the homilies and sermons they gave the people on this feast day, did not draw their teaching from the feast itself as from a primary source, but rather they spoke of this doctrine as something already known and accepted by Christ’s faithful. They presented it more clearly. They offered more profound explanations of its meaning and nature, bringing out into sharper light the fact that this feast shows, not only that the dead body of the Blessed Virgin Mary remained incorrupt, but that she gained a triumph out of death, her heavenly glorification after the example of her only begotten Son, Jesus Christ-truths that the liturgical books had frequently touched upon concisely and briefly.