The points at which the game transition between acts seem a bit arbitrary (mainly for Act I to Act II), and I don’t see a narrative or mechanical reason to lock us out of previous maps and quests. As far as I remember, previous Baldur’s Gate games didn’t have this kind of points of no return. Why do you think they did it? Do you like it?
I’ve heard of this early ending and yeah, maybe that’s it because I was able to go back to the very beginning as far as just before entering Rivington. After that, is when the only FT point I had was my camp. I did all the top of emerald Grove, then the underdark, then the mountains, then the shadow forest before finally moving on.