Is it permissible to cover the face with the niqab during Hajj?

Concealing the face for women is a good conduct, but during prayer, it is not permissible to cover the face except in the presence of non-Mahrams. During Hajj, it is not permissible for a woman in a state of Ihram to wear the niqab (face veil), but she may lower a covering over her face if it is not tailored, unless no alternative to the niqab is available and a man to whom she is not lawful to be seen by is present. The proof for this is the hadith from Abdullah ibn Umar, may Allah be pleased with them both, "The woman in Ihram should not wear the niqab, nor gloves."

As for lowering a covering over the face, other than the niqab, out of fear of foreign men, this has been authentically reported from some of the female companions. A'isha, may Allah be pleased with her, said: "We used to go out with the Messenger of Allah, peace and blessings be upon him, while we were in Ihram, and when riders met us, we would lower the garment over our faces." Sheikh Ibn Uthaymeen stated that the Prophet, peace and blessings be upon him, did not forbid covering the face but only forbade the niqab. If a woman in Ihram covers her face, there is no harm in it, but it is preferable to uncover it unless there are foreign men around her, in which case it is obligatory to cover it.

Regarding your past Hajj, there is nothing upon you due to the excuse of ignorance. Whoever commits a prohibited act during Ihram out of ignorance or forgetfulness incurs no sin and owes no expiation, as stated in the hadith of Ya'la ibn Umayyah, where the Prophet, peace and blessings be upon him, commanded a man who had entered Ihram for Umrah with his beard and head stained yellow and wearing a jubah (cloak) to remove the jubah and wash off the yellowness. Sheikh Ibn Uthaymeen mentioned that ignorance is like forgetfulness and compulsion; whoever commits a prohibited act out of forgetfulness or ignorance has nothing upon them, but they must remove it as soon as they remember or become aware.