I fully agree with you. However, early lessons always mark its use as incorrect. For example, "where is mother?" is not "де є мама?", but "де мама?". Frustrating to learn I've been speaking wrong all my life. lol Actually, I've always considered it as optional, but more formal speech.
Not native, but it is my first language. However, like many/most of my Ukrainian contemporaries English was our preferred language. So while my speech may be acceptable, but not perfect, reading and writing is not. The last point in my post was that the curriculum developers do not consider it correct.
Native speakers created this course, so it is easy to guess what they think :) . As «у когось є» is found in literature and poetry of the last two centuries, I feel the creators had good reasons to consider it appropriate (at least for everyday Ukrainian)—even though the expression probably did originate as a calque from Russian.