Neither ḥmyˁ nor ḥmyr is "yeast" per se as in some other lexica, but rather, as described in the citation from Isho'dadh cited above, leavened dough and/or starter leavening.
The strange but well established Samaritan usage corresponding to standard Hebrew שאר "flesh, relative" would appear to result from a confusion with its consonantal homograph שְּׂאֹר "leaven", which are pronounced the same in the Samaritan tradition.