Is the mother's paternal aunt, who is considered to be in the position of a grandmother, a mahram to an adult man, and is it permissible for him to uncover his face in front of her?

The mother's paternal aunt is considered a mahram to the children, and it is permissible for her to uncover what is permissible for mahrams to see, and she is treated as they are treated. The general rule is that the relatives of the father or the mother, no matter how far up the lineage, are related to the children by the same degree of kinship.