Dear Dr. Mak,

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. 
​I assume so, though I am ignorant about the history of the Vṛddhayavanajātaka.

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?
 
​I think that this mixture is preserved in some older strata of Buddhist scriptures and epics by chance, while, in other later texts, either verses
 were composed in pure 
Triṣṭubh
​-metre or in pure 
Jagatī
​ metre from the beginning, as in the cases of ​
the Avadānaśataka,
​ ​
Divyāvadāna
​, ​
Udānavarga
​,
Mahāyānasūtrālaṃkāra
​ etc., or the mixed verses were standardized by later redactors, as in the case of the Gilgit-Nepalese recension of the Saddharmapuṇḍarīka. To my opinion, in early times, this mixture was 
probably considered as stylistically elaborate
​ --- therefore not hyper-metrical​
, but later it
​ came to be regarded as odd. By comparing Sanskrit fragments from Central Asia, which generally preserve older readings, and their parallel (newer) Nepalese manuscripts of Buddhist scriptures, one can trace this shift.
 
With best wishes,
Seishi Karashima