PWG i-umlaut

Hi Rua ! Did i-mutation for 'e' carry over into PWG from PGmc ? For instance, I see that in the *jestu (“yeast”) Inflection table that the Dative singular and Nominative/Accusative plurals have *jestiwi instead of *jistiwi. Is *jestiwi correct for these cases ?

I'm not sure, but I vaguely remember Ringe saying that alternations within a paradigm were eliminated. Maybe it was levelled in different directions though in different languages?