some kind of weird bias against transmascs

It's this one, lol. Most trans men don't want to hear about how reductive misogynistic stereotypes like "no girls on the internet" still apply to them too.

Our culture has a suspicion and hatred of people assumed to be assigned female at birth who deviate from female gender roles and the LGBT community isn't really exempt from that.

Unfortunately even other trans mascs aren't exempt from this and will sometimes police or nitpick each other, accusing each other of "toxic masculinity" on trumped up charges... like, if a trans guy doesn't want to wear nail polish or something, or if he doesn't want to have kids or take care of kids, it's because of toxic masculinity and not just because he's a guy.

This is not to mention the fetishism and genuine danger faced by younger trans guys getting groomed online and so on. The end result is most trans men and mascs pretending to be cis dudes online.