What is the ruling on a woman covering her face during Tawaf in 'Umrah, and does this contradict the uncovering of the face and hands during Ihram?

It is true that a woman is commanded to uncover her face during the state of Ihram, and this uncovering is required if she does not mix with strange men (non-mahram). However, if she does mix with them, then she is commanded to cover her face, as narrated by Aisha, may Allah be pleased with her.