“Mystery” about the Purification in the Extraordinary Form

From what I can tell, the chant “Exsúrge, Dómine, ádjuva nos”—which formerly was sung while candles were being distributed, toward the end—was removed in 1962. It does not appear in the official 1962 Missale Romanum. Nor does it appear in the 1961 Solesmes Graduale Romanum. The Liber Usualis has it, but it’s well known that Solesmes did not update many rubrics in that book, such as the “Feast Ranking.” As far as I know, this deletion has never been mentioned (or explained) by any author.