It would be a great bit of characterization if it was intentional, plus it would leave a path open for goody-two-shoes type characters to romance Astarion in spite of not having earned massive amounts of approval.
I wouldn't be surprised if it is, though!
In most popular CRPGs of modern years, like Pathfinder, PoE and DOS2, your approval is not the only thing that matters - the way you approach your companions matters too (aka. "speech checks" or "dialogue checks"). A few examples that most people probably know of would be: Tristian (Pathfinder), Pallegina (PoE1 "romance"), Aloth (PoE2) and Lohse (DOS2) are great examples of NPCs that responds poorly if the PC is pushy - they need the PC to give them some personal space in order to be properly romanced, regardless of your approval levels. Other characters, like Tekehu (PoE2), Ifan (DOS:2) and Regongar and Octavia (Pathfinder) respond better to more straight-forward approach. In my opinion - and probably most peoples opinion - this makes perfect sense and it keeps the characters true to their nature.
I would be genuinely surprised if Larian chose to move away from this system completely and relied 100% on approval levels, considering they partly used this system themselves in DOS:2, although it is not as... Punishing to choose "wrong" as it might be in the other titles mentioned (maybe Sebille is a bit more picky, I have not had a chance to romance her yet since I ... Uhm, well I killed her :x Imma bring her on my next playthrough though). Most characters have some kind of more-or-less difficult / important "speech check" before romance can be triggered and Astarion and Shadowheart strikes me as two characters that probably would have a little bit more difficulties opening up emotionally, compared to the rest of our companions, and hence I'd imagine that they will have a couple of speech checks in order to progress our relationships properly with them at full release.
