Unfortunately, I’ve reached the limit of blind guessing. There’s too much specific information about your project, which I don’t know. Naturally, there are more knowledgeable user here, but I can’t image they wouldn’t also benefit from knowing more.
What BP is this event beginplay running from? An AI enemy? Your main character?
If AI, is it preexisting or spawned later?
Is BP Duel Character preexisting or spawned later?
Do you begin with a widget (system) menu or just go straight into play?
Have you confirmed that Get Actor of Class consistently sets BP Duel Character (via print strings)? And/or, have you isolated the exact node that doesn’t consistently fire (again, via print strings)?