Do you know Feng Shui? Next to the ancestral grave is my grandfather's grave, and then one or two meters to the right is my grandfather's grave. My grandfather's grave is higher than his ancestral grave.
After all, it is reasonable that the ancestral graves have not been filled by future generations for a long time, or have become shorter due to perennial rain erosion. Since you have such concerns, you might as well put stones around the ancestral grave and add some new soil. Stones are built to prevent rain from washing away. In fact, it's nothing special to add new soil to the grave, and Tomb-Sweeping Day didn't have to add it that day.
In addition, many regions and people are used to pressing a piece of paper with stones on the tomb during the Spring Festival holiday, which means that this tomb is often visited by future generations. But that will directly affect the fortune of the next generation, which is called luck in Feng Shui. The correct way is to grab a handful of soil next to it and hold down the yellow paper.