This is where the crux of all the perceived inconsistency comes from. The design choice to not only have all the companions flirt with each other, but to to have cases like Karlach volunteer themselves to be involved only to heelturn on it later because of a general lack of development. The entire dynamic that was advertised to us got pinned on Halsin doing an absolutely terrible portrayal of it, and signs of a better dynamic that DOES involve the other origins they didn't get to develop it for remain leaving us all disappointed with the dynamic we were left with.
You got it entirely right, but that's why it's not "perceived" inconsistency, that's why it IS inconsistency. They, for reasons only they know, decided to have all origin companions be mono. Any kind of flirting they did with each other becomes irrelevant by the time you're nearing the end of act 2, because you are forced to choose a companion, meaning their character development has reached the stage where they are no longer interested in each other. At least not together with you. They want you, or nothing.
If they chose to have some of the more obvious choices be poly (Shadowheart/Karlach/Tav or Astarion/Lae'zel/Tav) then there wouldn't be a problem anymore. But they didn't do that, they instead wrote them all to be mono and gave them mono romance progression. Thus, the inconsistency when Halsin suddenly shows up.
