it changed its meaning because bad actors decided to use that term specificlly, to imply that "trans[gender]" is a third sort of option (that is not part of the "normal" or "real" genders) and also the biggest enemy of "real [gender]" and especially "real women". The same way "transsexual" became a slur, the same way "retarded" or "special needs" became slurs/insults. If you say someone's name with enough hatred in your voice enough times it'll become an insult as well.

it's stupid i know, and we should reclaim it at one point, but for now it's a dogwhistle for transphobes, at least in online spaces