This work presents a prediction of a coherence-loss cutoff in ultra-high-energy cosmic rays (UHECRs) and astrophysical neutrinos derived from the Physical Time (rho_t–k) framework. At extremely high Lorentz factors, relativistic spinor modes lose phase coherence due to gradients in the Time-density field rho_t. This mechanism produces a smoother and earlier suppression of the UHECR spectrum than expected from the classical GZK interaction and introduces energy-dependent damping in neutrino flux at multi-PeV to EeV scales. The predicted effect applies to both protons and heavier nuclei, with stronger suppression for higher-mass compositions. Neutrinos are also expected to exhibit coherence-related damping above several PeV, independent of weak-interaction cross sections. Current observations by the Pierre Auger Observatory, Telescope Array, and IceCube already operate in the relevant energy range, and future missions such as POEMMA and IceCube-Gen2 will further test and constrain the predicted cutoff behavior.