Astarotte's Toy Season 1 Episode 11

One morning, Lotte sees the leaves of the world tree in a rare color. She also finds a mark on her back which looks like one of the leaves. Judit interprets this as meaning she'll grow her wings soon, a sign of adulthood. However, Mercelida explains that the marks of adulthood form a heart shape. As it turns out, the world tree is rejecting Naoya and Asuha for upsetting the balance of magic in the Monster Realm and has affected Lotte's development, as well.