Vowel lengths of comparatives in -ίων

Does someone know a rule for the vowel lengths of comparatives in -ίων?
There seems to be discrepancy as we have
κακῐ́ων
but
βελτῑ́ων.
Now when is the Iota long?

Ἦ τις γνωρίζει νόμον τοῦ χρόνου τοῦ ι ἐν τοῖς συγκριτικοῖς εἰς -ίων;
Δοκεῖ μοι ἀσύμφωνον ὅτι τοῦ μὲν
κακῐ́ων
βραχύ, τοῦ δὲ
βελτῑ́ων
μακρόν. Πότε δὴ τὸ ι μακρόν;

The iota is normally long; occasionally short in Homer but never in Attic.

Thank you :slight_smile: