I think part of the problem with some of Tav's facial expressions stems from the fact that there are two valid interpretations (at least) of the romance with the Ascendant. One is that Tav was naive, expected Astarion to remain unchanged, and essentially feels duped/trapped in a relationship as his spawn. The game certainly provides dialogue options to support this. However, there's an equally valid way to roleplay it in which Tav made their devil's bargain happily, accepts the power structure, and enjoys how domineering the Ascendant is.
The problem being is that the game only bakes in one set of facial expressions in cinematics. So while we have dialogues to roleplay either story, we don't always get graphics that line up. I do agree, though, that by this point if you agreed to be turned and didn't chose to break up then your Tav is probably into it and shouldn't look scared/angry about it.
