How tf is "transsexual" offensive to folks now?

"I was transgender before I medically transitioned and now I'm transsexual." This comment has caused a lot of backlash from younger and older trans people in my life, and I can't quite understand how. I have truemed friends who think I wasn't transgender before I medically transition, and then I know young trans folks who tell me the terms I use for myself are "offensive." Wtf is up with the community that this is offensive? I am lucky that I have been able to medically transition, but it didn't mean I wasn't trans before I had these opportunities. Maybe the feedback has been from damaged folks (arent we all) and I am not meant to make sense of it. When did this change come about making the term I see as accurate turn into an offensive term?