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?
Odd, I tried to go back to Act 1 from Act 2 (looking for a vendor since spoiler stuff happened at the only settlement in cursed lands) and when I clicked the elevator it told me I “really shouldn’t” like twice and the third time it game over’d me in a unique and creative way… I felt that was a pretty hard lock out of act 1… Lol
I think there is a grace period after hitting act 2 where you still can go back but once you’ve done a bit of story related stuff you are pretty locked in.
You could try just opening the map and bringing up the list of fast travel spots. This will let you fast travel even to points on another map. Like if you’re in the under dark, you can warp straight to Emerald Grove without going back up topside. I had to do this when in the basement of Moonrise Towers because I wanted to go back and sell shit before moving forward, but there’s no transition back the way you entered.
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.
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.