I have had similar experiences. My closest LGBT friends (one gay cis man, one ace trans woman) don't do this, but I have run into it. Also, mostly AFAB queers do it (in my experience).

I have a lesbian masc-nb friend (who is awesome) but who kinda used to do this. We had some really long talks about it and she said that when she first heard me say that trans people are not Gay+ she thought it was funny but didn't really get how anyone could think that. Then at some point she had a reckoning when she really realized what I meant by that and that somehow in her mind she had been thinking of trans men as on the lesbian side of the binary and not on the men side without being aware of it.

This is tricky because in someways it's a problem with over-relating and assuming more similarities than reality provides. On the surface, we appear quite similar (masc AFAB) but in some invisible, but important ways we are very different. First, I'm (mostly) gay and phalloplasty has been delayed due to some issues and then corona which has been really hard for me. She does not want a penis and does not like penises and has no interest in ever interacting with anyone else's penis. And second, I can exist in all-male spaces and am interacted with as a man in day-to-day interactions by the vast majority of society. When I'm in a group of all men, there are no women present. She has never and never will have those experiences. In the beginning of our friendship, I think she was just really happy to meet someone similar to herself and we got on really well together. It was easy for her to see and experience the similarities between us and not notice the differences.