We investigate the spontaneous emission of a two-level system coupled to a photonic waveguide, showing that dynamical dephasing
in the photon modes profoundly alters the decay law. In the absence of dephasing, the emitter displays conventional exponential decay
followed by a long-time power-law tail – observable only at extremely low survival probabilities. Strikingly, when dephasing is
introduced, a robust power-law decay emerges already at short times, driven by photon diffusion in the dynamically disordered environment
rather than spectral edge effects. These results reveal a novel, decoherence-induced mechanism for non-exponential spontaneous
emission in waveguide QED platforms.