Science can't explain everything, and there are many things that can't be explained by science but can happen. The ancestral grave is very particular about geomantic omen, so it can't be destroyed, otherwise it will break geomantic omen.
If it is a cemetery, it can pass the appraisal. If it is man-made damage or quality problems, you can ask the management department to repair it (the other party has not fulfilled its duty of care). If it is a natural disaster or natural weathering, it will only be repaired at its own expense.
If it's not a cemetery, spend money to repair it.