I agree with this so much - the end of act 2 was such a narrative high I was excited to see how they topped it!
And then they didn't. And then I stopped levelling. It wasn't the climax to the story I was expecting.
I always struggled to describe what was so disappointing but you hit the nail on the head. Starting act 3 felt like I was sent in time back to act 1 - aimlessly wandering around the map until finally stumbling across the next main story point.