It's such a contentious topic because bodily dysphoria is well-documented, at least casually, among butch lesbians. It's not weird at all, in lesbian circles, to be a butch who doesn't want her chest or genitals touched, or actively seeks aggressive breast reductions, or goes exclusively by a male name, or refer to themselves as "guys", or any combination or extention of the above.
Some pass as men and still call themselves butch lesbians. So I guess when you're that far down the rabbit hole, what you label yourself as is a matter of semantics more than anything else.
I'd guess at least a portion of FtMs are similarly afflicted because positive butch role models are rare these days.
True, actual butch lesbians of that type definitely exist. The problem is that the current "he/him lesbian" crowd are, of course, not like that at all. They just want to play the genderfluid pronoun game, and look about as masculine as any of the other girls on tumblr. The fact that a lot of that group considers "butch" to mean "wears flannel sometimes" is probably par for the course by now.
It's a popular tumblr thing, pointing at a tiny minority or single example of something and saying "See? It's real. So I can do it too". Even if it doesn't actually apply to them, they insist that if you criticise
them, then you're also criticising the unarguably legit people (who they copied the idea off in the first place). Therefore your criticism is invalid.
See also: (gay) men who are effimate or cross-dress ("See? So transmen can do it too and are just as valid!"), autists who use stim toys ("because I'm autistic, you can't think it's weird that I wear a chew toy in public even though I otherwise function normally"), mixed or otherwise non-white people who look white at a glance ("I'm 1/16 native so I am POC and oppressed even if I don't look it"), and so on and so forth.
It's basically a hive of "Oh, I can apply that to me and get oppression points" combined with a lot of them going through those teenage years where everyone things they're more special or hard done by than the rest of the world. I guess the real problem is when they don't leave that crowd, stay surrounded by the same people, and keep believing that shit well into adulthood.
The fact that there's an attitude of "you're not allowed to have an opinion on X thing unless you're part of X group" probably doesn't help, either, especially if you need to be part of the woke crowd to fit in.