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?
The x amount rest or in game time is how it worked in pathfinder and stressed the hell out of me in those games. I’m happy they give you an alert in BG3 and let you know to go back if you want to.