Here in Italy (that is where the word was invented in the first place) that is considered a bad common mistake. Forgetting about the etymology or misusing a word is not what I would define as "evolution". We say multi-media, mass-media ecc. for a reason, for the same reason we don't say multi-mediums or mass-mediums.

To be pedantic, the correct grammatical rule in italian is that foregin words (including latin) are kept invariant between singular and plural, so the plural of medium would still be medium.

This rule is of course often misapplied, especially for latin words.

That’s the thing, in English when enough people make a mistake it stops being a mistake.

Though English has two plural forms "media" and "mediums", are they interchangeable? If you say "media" I think of more than one medium of communication, but if you say "mediums" I think of a group of humans who engage in necromancy and talking to spirits and stuff (being a medium as a job)

I believe it’s context specific. Both are in common use in acoustics or bacterial cultures, but it’s electronic media and spirit mediums.