The same figure occurs in the introit Laetare at satiemini, and in Cogitationes, which is borrowed from it, at eorum. The first three notes are long in all of the notations, yet I hear scholas that claim to use the Solesmes interpretation sing it long-long-short, long-short-short (you can hear this in the recording from Triors), short-long-long, or even short-long-short! The modern notation Liber also has all three notes marked long:
I know there's some wiggle room for interpretation, but I can't see justification for anything except three longs here. For a Solesmes-style interpretation, I hold them about 1.5 length, not double. Does any Solesmes method authority (Mocquereau, Gajard, Suñol, Carroll, Ward, etc.) advocate the long-long-short or long-short-short interpretation?
Yet the recordings from Triors sound to me like a lengthening only of the first note, the normal short value already regained on the second:

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