The Beauvais setting merely has doubled notes, but some of the comparable three-part congeners (and once in Beauvais) have the second of the doubled notes as a liquescent.
We know that the cephalicus started life as a liquescent neum, its second element emphasizing the pronunciation of a doubled consonant or the like (as a sernivocalis), but it is not always clear as to whether this meaning still obtains, or the second note is merely a species of fractio modi, or whether the symbol has atrophied to its later meaning as a longer note.
the syllable in question is marked in bold) motive that recurs as a species of Leitmotif: this phrase is written without any liquescent at its third appearance (before "Ut scribentis"--also subsequently, with one exception), whereas the first occurrence seems to carry a one-note liquescent, the second (in common with one later appearance) a cephalicus.
It has a typical crux relating to the liquescent forms alluded to earlier and whose understanding is an essential prerequisite for a new transcription of the work (and of whose details I failed to take proper account in the 1976 Plainsong and Medieval Music Society edition).
The symbol should have been a single-note liquescent.