This speaks to the strength of the shaman's people and their culture.
The culture survived 600 years without being conquered by other tribes or being driven from their land.
Since memory of the shaman and the location of her grace had to be passed on by word of mouth, these people had a strong sense of self and a strong faith in their belief system.
I am guessing that the grave was well marked and was maintained for those 600 years and that when these people were finally driven out or subsumed in to another culture that the grave marker material was stolen for other purposes.
Or, they didn’t know there was an earlier grave.