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?
Once you hit the lock out point, there aren’t any waypoints for act one when you open the map… I didn’t get all the way through acts one and two by literally walking everywhere and ignoring fast travel, lol.
No waypoints was why I had to walk back to the elevator to even try going back. I don’t want to drop spoilers, but I have a hunch what the “point of no return” is and depending on your methodology playing the game you might trigger it relatively early in act 2 or it could be very nearly the last thing you do… I’m assuming you happen to have done the later and didn’t even notice when you got locked out of act 1.