All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The correct answer, if Allaah Willed, is that unveiling the face of the deceased at the time of burial is not from the Sunnah; instead, it is in contrast to its apparent ruling.
In the Fataawa of the Permanent Committee (of Saudi Arabia), it is written:
We do not know of any evidence that suggests uncovering the face of the deceased person in the grave. Rather, the apparent ruling of the legislated evidences indicates that the face is not to be uncovered, and males and females are the same in this regard. This is because the basic ruling is to cover the face along with the rest of the body, unless the man is a Muhrim, in which case he is not covered.
While the person who asks this question was wrong for doing what he did when he buried his father, he is excused due to his ignorance and due to the good assumption he had about the gravedigger who guided him to this act.
It could also be of some help to know that some of the scholars had much to say on this topic. Shaykh Ibn ‘Uthaymeen said in Liqaa’aat Al-Baab Al-Maftooh:
Uncovering the face of the deceased person when he is placed in the grave is the act of some of the Salaf, so whoever does so is not blamed. However, the whole face should not be uncovered but only the cheek that rests on the ground. There were also those of the Salaf who did not uncover the face. Hence, this is not a strict matter.
Allaah Knows best.