It seems that death eaters have “anti-time-looping wards” (chap 94), we don’t know exactly how they work, nor if they were used in the graveyard, but it could very well be that no time-traveler can check the events that happened on the graveyard.
I myself will guard this place until six hours have passed, for I do not fully trust the wards I have set; and four of you shall search the surroundings for signs of anything noteworthy.
It seems that death eaters have “anti-time-looping wards” (chap 94), we don’t know exactly how they work, nor if they were used in the graveyard, but it could very well be that no time-traveler can check the events that happened on the graveyard.
They were used. Voldemort mentions not completely trusting them.
Edit:
Quote from chapter 113: