That’s easily fixed by the Light forsaking Calia sometime during Shadowlands and her becoming Natalie Seline pt 2, and maybe giving Natalie Seline a Forsaken model of some sort and make her a Forsaken character like she should’ve been.
Easy enough to do. We know from Alpha questing that the Light and Void have both tried to invade the Shadowlands. Have her be told by the Naaru to usurp the Shadowlands given she is Undead, and let her break from the Naaru as Sylvanas broke from the Lich King.