The way I see it, the player character on that path is currently portrayed as unaware of the change in Astarion's temperament ascension would bring with it, and does not want to break up if it weren't for that change.
The period between being turned and the ending scene is one of profound unease and growing suspicion. Yes, even if you never pick these dialogue choices, the game decides you're just not letting these feelings show. Your feelings are decided for you. That usually happens if a lead writer has very strong opinions of what's going on, and they're not well translated.
Because of limitations of the engine and the late addition, the overplayed emotions in the kissing scene and some of the not so matching romance pathing are rather jarring. In a way, you're supposed to imagine the player character is on edge now all the time, but it's only visible in the kissing scene. This is objectively not well done. However, I have strong doubts Larian would do this without motivation. It's expensive to create such scenes. It was meant as a statement on how the player character *ought* to feel, which is ironic when one in game path is living in blissful ignorance of how Astarion would actually react when defied.
You can see the change immediately after ascension. He even lets Tav know he can feel their hearbeat and can tell they want to be dominated by him and that everything's changed. He's clearly telegraphing that the relationship will become different from that point on. Tav would have to be in complete denial to not see that. During the long rest afterwards there are plenty of choices to criticise him and zero positive replies about the ritual you actually helped him with. The breakup options are all about not wanting to be with him due to the change in his behaviour. There is no growing suspicion, only Tav hating his new self, liking what they see or believing he can still eventually be a good guy since he's supposed to have human emotions now.
In the same patch they added extra, hidden lines for the situation when Tav abandons him and accompanies Karlach in the dock scene. Upon reuniting with him in the epilogue Tav says: "I'm sorry I kind of disappeared. I didn't mean to leave you like that." Doesn't sound like someone who is fearful of him and wants nothing to do with him.
During the epilogue party you can be a perfectly harmonious power couple. Most dialogue options are positive. Even when you're not happy in that scene, Astarion tells you he's putting big effort giving you the best life he can. They sound more like a bickering wedded couple fighting over one thing they disagree on rather than someone fighting for their life and wanting to run away from their captor. If Tav was so awfully mistreated they would complain to their companions and ask them for help.
The only one deciding how my character ought to feel about another character is me, period. It's my character's story not the writers'.
