It’s like “ body “ fluids “ vs. “ bodily fluids “. I believe only the first one is correct.
I personally only started hearing the second one used about 30 years ago.
I'm afraid that belief itself is simply incorrect
and the accepted equivalence of "preventive" and "preventative" has been documented for over 300 years. It takes only a few minutes of searching on the web to find any number of references. It also takes little time to find various opinions about one of their being the "correct" one and the other "incorrect". But in fact these are merely opinions and not supported by the historical record or gold standard dictionaries and language use books over these centuries. Such sources include not only Merriam-Webster, but the
Columbia Guide to Standard American English among others. It's true that various historic sources have deprecated the use of "preventative", and this includes earlier editions of the revered
Fowler's Dictionary of Modern English Usage. But
The New Fowler's Modern English Usage, takes the view that both "preventive" and "preventative" are acceptable.
I do in fact have sympathy for your view -- and generally for a dim view of linguistic drift. But it happens and there is little to stop it (especially now as English is taught in many public schools). My own habit has been to use "preventive" as the adjectival form and to use "preventative" as the nominative. But even that, it appears, is a somewhat arbitrary distinction. Some aficionados of the
Chicago Manual of Style seem to favor it, but others don't. However, even in that context the two forms are acceptable and considered to be semantically equivalent.
Of course none of this means that we as individuals can't enforce our own style and usage principles to view "preventive" as correct and "preventative" as incorrect. But it does mean that we don't have any objective and authoritative grounds for complaining how other people use them.
Sometimes, it's just hard being a curmudgeon, and you just have to take the hits.