The suffix -izm is borrowed from Greek via Latin, it's not native Polish, but in the Polish native words z is palatalized to ź [ʑ] before a palatalized m (mi/mie), cf.:
wezmę ‘I will take’ with z before a non-palatalized m
vs.
weźmie ‘he/she/it will take’ with ź before a palatalized m,
so it's natural for a Pole to pronounce ź [ʑ] also in the borrowed suffix -izm in the locative case -izmie, and both pronunciations are considered plausible in Polish, although in spelling only -zmie is to be used. See this Wiktionary article on the Polish suffix -izm which has a footnote 1 in the declension chart saying that while the spelling is only -izmie, the pronunciation can be both -izmie and -iźmie.
In Russian the situation is pretty similar, the Soviet leader Khrushchev, who wasn't quite aware of the standard pronunciation norm due to his very low level of education, was famous for palatalizing z in the Russian words for “communism”, “socialism”, and “capitalism” in the locative case, which is quite often exploited in the jokes that parody his way of speaking, Russian grammar and pronunciation being very prescriptive with the palatalized [zʲ] not allowed in that environment.
Also, you might want to have a look at the discussion of a very similar question on Reddit: Pronunciation of "-zmie".