No, it is not an exception.
Mostly, Italian words are descendant of Latin words, even better their ablative forms.
For the words belonging to the first declination, usually feminine, the ablative singuar and plural endings, respectively "-a" and "-is", become "-a" and "-e" in Italian.
For the words belonging to the second declination, usually masculine, the ablative singuar and plural endings, respectively "-o" and "-is", become "-o" and "-i" in Italian.
For the words belonging to the third declination, both masculine and feminine, the ablative singuar and plural endings, respectively "-e" (also "-i" somewhere) and "-ibus", become "-e" and "-i" in Italian.
About "cenere", from the Latin feminine noun of the third declination "cinis/cineris" (ablatives "cinere" and "cineribus"), regularly it takes the plural ending "-i".