Stan Belik, regarding your question: I don't think it's a bug - no one seems to have encountered such bugs before; it's more likely that the campaign authors introduced a powerful hero in this scenario, whom you already played in the previous one; so you can use him, but if you don't want to and can win without him, please do!
And if you check like this: dismiss Haart, and then proceed to the next scenario - will he appear with the same initial skills and artifacts?
(Василь Симоненко. Люди – прекрасні)