We investigate proton acceleration in the relativistic transparency regime using ultrathin foils irradiated by petawatt-class laser pulses, achieving proton energies exceeding 80 MeV. Shot-resolved analysis of transmitted spectra reveals that temporal gating at the onset of relativistically induced transparency (RIT) shapes the pulse, producing either smooth or modulated spectra. A spectral smoothness metric, benchmarked against PIC simulations and a semi-analytical gating model, demonstrates that different transmission levels correspond to distinct transparency onset times. We find that maximum proton energies are consistent with transparency occurring near the laser pulse peak, while later or absent onset can also drive efficient acceleration. These spectral diagnostics establish a powerful probe of acceleration cascades and a route to understanding and controlling shot-to-shot fluctuations in laser–ion interactions.