Not sure what your facepalm is about, read the book I cited
Kirsch, C., Enchiridion fontium historiae ecclesiasticae antiquae. ED. 4 Friburgi, 1923.
it contains all of the letters of Pope St. Leo II, 3 are to the western Churches, one is in greek(translated into Latin) to the east.
The Third Council of Constantinople also states this, as does the 14th Council of Toledo which also affirmed the ruling held at Constantinople.