Ocean Explorer map fails to locate ocean monument

Map was bought from a villager. Initially the map was completely white. After quitting and reloading, it loaded as a normal looking explorer map, indicating an ocean monument. There was a tiny white dot in the upper right corner indicating the player was very far away from the monument and needs to travel south west. After travelling to -10000, 10000, it still has the dot in the corner. Examining the level.dat file shows there are lots of monuments in the world and not all as far at 10000.

Update: after quitting world and logging in, map now shows dot in bottom left corner, and a some blue indicating I had passed by the monument, but the map failed to update.