Might also be my interpretation because his overheard dialogue is mostly him being nostalgic, annoyed that there isn't any blood on the menu or him worrying about the other spawn, if you went down that route. All of which, lack of substance aside, stand in contrast to his mood during the actual dialogue.
IMO this is less of an overheard dialogue issue but rather a side effect of the dialogue for the spawn route beating you over the head that it's his good ending and the acting's believability often not meshing well with the over-the-top affect they've decided to stick with for most of Astarion's scenes (criticising parts of the acting is surely controversial, but I stand by it). The result is that I can read it as intended as completely genuine by the writers and the actor, just not well executed.
For a well executed, good moment where Astarion sounds completely genuine in his hope and contentment but the dialogue is also not sugarcoating the reality of the horror he's gone through and the future consequences of that, thus making it more real and nuanced, I'd go directly to the graveyard scene. In which, curiously, he drops the affect.
