Well, it's could be a couple of things. First might be familiarity. Honorifics tend to fade when people get close. Second could be because both you, and ser Alberic, are speaking from above her station. The WoL is an azure dragoon that was recognized by the eye, so, despite not being part of the Ishgard standing army, he is still considered at the pinnacle of the Dragoon order. Sometimes people don't use honorifics when addressing someone beneath their station. Lastly, it could be a bit of an oversight caused by plot related events.
Spoilers: In the Drgoon questline
Heustienne ends up being skyjacked by a dragon and, essentially, kidnapped. While in captivity, she is forced to drink blood of a Dragon. During this time she is considered MIA, but presumed dead, but if she was ever found out she would have been excommunicate her from the Dragoon order and likely killed by the inquisition. Knowing this, she essentially excommunicates herself.
It's possible that because of these events, she is no longer referred to as "ser," and her knighthood is revoked... though, considering the only parties involved who knew about the truth of these events were the WoL, Ser Alberic, and Heustienne's father... It seems a little extreme that they would no longer use her honorific, unless she herself discouraged them from using it. She seems to be her own worse critic, so that's also possible, but I don't recall there being any evidence of it in cutscenes...