In game 9679433 players were not getting +5 VP from Keep when buying a treasure from the Black Market deck.
From my own experience, Keep+Black Market usually works. I loaded your game at the end and Horn of Plenty does indeed count it for Keep points.
Maybe you missed the 5VP during the game being added? (when I bought it from Black Market, the VP counter jumped from 8VP to 13VP corectly.)