Is someone who, out of ignorance, committed some of the prohibited acts of Ihram during his Ihram, then later discovered that they are among the prohibited acts, considered excused, such that no expiatory offering (fidyah) is due upon him, or is it due upon him?

Since the doubt has arisen in your mind that these things are among the prohibitions, you are considered negligent, and therefore the fidyah (definition) is obligatory for you. The ignorant person is excused because of their lack of negligence, but if they are negligent, the fidyah becomes obligatory for them. This is more cautious and absolves one's responsibility, and it is a way to avoid scholarly disagreements.