https://www.amazon.com/Missa-de-angelis-Agnus-Dei/dp/B00Q3WAWMA

Why would they chant the Agnus Dei to the tone of the Sanctus? They don't in this recording.

All I know is that in the 1934 Graduale Cisterciense that is the melody used. (TBH it doesn't really bother me much since the de Angelis Mass is, as the Solesmes monks say, "inauthentic" any way.) I have heard that the Cistercians at one point during the time of the Solesmes chant restitutions wanted to ditch their own chant books and go with the Benedictine/Roman books, but that a monk from Solesmes (Gajard? Pothier? Mocquereau?) urged them to keep their own chant tradition.

In this* new Kyriale Cisteciense (2000), here, they have added the rhythmic signs of Solesmes (dots, horizontal episemata) and have added in quilismata - which do not exist in earlier Cistercian chant books. They have also adopted some melodies according to the Roman books (both the Graduale Romanum and the Graduale Simplex!). Personally, I view this as a loss, not a gain, as the peculiarities of the Cistercian tradition are wiped away in favor of a kind of Roman conformism. The chant has NEVER been completely uniform from one place to another, there have always been regional differences, and they should remain!

And, frankly, while I'm at it, a pox on any Anglican or Anglican-Use community that adopts the Roman Kyriale or Graduale. They should use the Sarum books, or books based thereon, whenever possible.

(End of screed.)

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* I say "this" because I do not know if this is an official book for the entire order, or if it is just a local book for an Italian Cistercian community. (I don't speak Italian, so I have no idea what the preface says.)

Except the Angli–Catholics largely never used the Sarum Gradual & the Ordinariate propers are based on those of the Roman Rite.

Is outrage! (With Oxbridge accent.)