Not that unexpected. Tsubasa's behavior and memories in the first chapter made it clear that she had some issues with her family, and had been treated violently in the past. That was clearly going to come up sooner or later.
Unless you mean the out-of-the-blue question about her being a prostitute based on speculation from strangers. What kind of world do these kids live in where two high school girls and a man walking together are assumed to be part of the sex trade? I've heard that teenage prostitution is common in Japan, but don't girls have fathers and other male relatives? Being accused of prostitution by random gossip seems to be a common plot device, so maybe it's just one of those things authors throw in for the sake of drama, or maybe high school gossip really is that awful.
Japan had a very bad problem with females joining the sex industri at some point. I dont know if it's still as bad
From Misaki's point of view, I can kinda see her reaction. After all, what she knows of Tsubasa is that she shows a very different face outside of school, seems street smart (compared to her) and accosted her for sex in a bathroom literally the first time they met in a casual setting. At the very least, she doesn't know much about her girlfriend, so anything's possible.