While Toby Fox, the developer behind both Undertale and Deltarune works on releasing more chapters, it's possible that players might not have discovered every secret within the chapters that already exist.
With the release of Chapter 2, players are eager to jump back into this magical world with Kris, Susie, and Ralsei. Chapter 1 of Deltarune features a secret boss that players might have missed on their first playthrough.