If this was the case for this rather difficult chapter of the Vṛddhayavanajātaka, this may then be interpreted as a vestige of an older version which somehow resisted standardization.
Is there any explanation for why this mixture came into being in the first place, especially in Buddhist texts and epics? If one considers the two meters as catalectic/acatalectic version of each other, and if one meter is predominant (upajāti in this chapter 66.3%, in the entire work probably 99.9%), wouldn’t the other be considered hypermetrical?