The relationship between cis feminists and trans women has always been complicated, and I’m going to say that realistically, there’s a generation gap at play. Not 100%, and of course there are common battles for all feminists of all ages and a lot we’ve never won. But broadly: earlier generations of feminists probably didn’t spend as much time discussing what a woman was, but fought really hard to establish what a woman was worth. And by necessity the approach had to be, “These are things we’re claiming rightfully for ourselves, and we’re not going to let anyone else swoop in and take them.”
On the flip side, a lot of younger cis women have grown up in an era when women don’t have to fight as hard in many contexts for certain basic rights, but are constantly being sent harmful messages simply about what a woman is. The feminist battle a lot of younger cis women know is: I don’t have to be thin, I don’t have to be pretty, I can be athletic, I can be into STEM, etc. It’s constantly fighting back against what feels like an attempt to define the boundaries of acceptable womanhood, so it’s a much more natural step for many of those cis women to view embracing trans women as another middle finger to the patriarchal idea that there’s a “right” or “wrong” way to be a woman.