This Technical Note (SLM-TN-2026-003) addresses the "surprising" energy gains observed in Plasma Wakefield Acceleration (PWFA) experiments at the FACET-II E-300 facility. Specifically, it provides a mathematical and physical resolution for witness bunch acceleration that appears to exceed the theoretical Transformer Ratio limits predicted by standard hydrodynamic and particle-in-cell (PIC) simulations. The paper introduces the Symmetric Layered Model (SLM) solution, proposing that ultra-high gradients within the nonlinear plasma wake induce a "Stueckelberg Shielding" effect. This effect momentarily decouples the Standard Sector electron from its Dark Sector symmetric partner, leading to a temporary reduction in the particle's Effective Metric Mass Key Highlights: The "Tether-Break" Mechanism: Explains how extreme plasma gradients strain the inter-sector phase-canceling relationship at the 145 MeV Stueckelberg portal scale. Mass-Reduction Formula: Provides the non-linear derivation for as a function of the local wakefield gradient Transformer Ratio Violation: Demonstrates why lower effective mass results in a non-classical acceleration surge, resolving the discrepancy between HiPACE++ / ImpactX simulations and experimental reality. Experimental Verification: Outlines a specific "Gradient Scanning" methodology to detect the efficiency "jump" at the threshold. This note is the third installment in the SLM Diagnostic Series, providing actionable theoretical support for high-gradient beam physics and next-generation collider design.