Karnatakabhashabhushana 6.24

And there is optionally the vowel i in place of e.

When the rule for ending in -su applies, the substitute i optionally replaces the vowel e.

kisuvol;

kisusañje;

bisunettar;

bisusuyil.

By fixed option, in some instances e alone remains, not i:

kēsuri;

kēsaḍi;

kēsakki;

kēsugur.