<p>While replica symmetry breaking (RSB) was originally formulated in spin-glass theory, its photonic counterpart has been realized in the last decade. The existence of a photonic glassy RSB regime has been demonstrated in several photonic platforms characterised by high disorder, such as colloidal random lasers and random fiber lasers. However, the emergence of RSB phase with weak disorder in a standard passive mode-locking regime has not been experimentally demonstrated. Here, we report such experimental demonstration in an ultrafast fiber laser. In contrast to the photonic glassy-like RSB phase in random lasers, the intensity fluctuations of the optical modes in the pulsating passive mode-locking regime originate from the interplay of the nonlinearity and competition of gain among modes, leading to different sets of activated and frustrated modes associated with each pulse. We study the underlying mechanism by characterizing the phase stability of pulses in the mode-locking regimes, demonstrating phase stability in the stable mode-locking regime, periodic oscillation in the pulsating mode locking regime with periodic pulses, and chaotic evolution in the aperiodic pulsating regime. We interpret the latter as an unobserved RSB regime in the aperiodic pulsating phase of passive mode-locked ultrafast lasers.</p>