Nostromo, Mr. & Mrs. Gould, Giovanni
Starts out strong and predictably Conradian: plot and character development perfectly executed, until last 20% of novel when Conrad drops the ball and goes for an easy predictable end.
In the time of Spanish rule, and for many years afterwards, the town of Sulaco--the luxuriant beauty of the orange gardens bears witness to its antiquity--had never been commercially anything more important than a coasting port with a fairly large local trade in ox-hides and indigo.
It was another of Nostromo’s triumphs, the greatest, the most enviable, the most sinister of all. In that true cry of undying passion that seemed to ring aloud from Punta Mala to Azuera and away to the bright line of the horizon, overhung by a big white cloud shining like a mass of solid silver, the genius of the magnificent capataz de cargadores dominated the dark gulf containing his conquests of treasure and love.”
Ayn Rand had the same problem with Atlas Shrugged. Her last 100 pages feel rushed; she turned three dimensional characters into one dimensional and shallow super hero comic figures. My guess whenever this happens in any novel, is that writers are rushed by their publishers to finish to meet a deadline, so they are not able to give it the quality investment of their time and thought to finished the ending with the same quality as the rest of the novel.