i've had a lot of those lately, including the goldfinch and the secret history, also a couple each of margaret atwoods and jonathan coes
I used to be constantly disappointed/ infuriated by the endings of novels. I just had to get over it and accept that it must be the most difficult thing for authors to get right. What I find more disappointing is when the decline begins within the final third or quarter of a book.
Years ago, every month or so I had to dodge Paul Auster novels, which upon completion, were hurled across the room in anger by my sister. She broke her favourite lamp once. The one that looked like a tree.