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 game warns you before you enter the mausoleum. There is even a character who warns you. That’s exactly how a dm would do. You even have 2 warnings actually. One when you enter the mausoleum, and one when you enter the final zone of it.