Spectral hardening revealed by geometric de-boosting in the masked jet of PKS 2155−304

Context. Blazar gamma-ray variability is predominantly stochastic and well described by red-noise processes. However, a subset of sources shows quasi-periodic oscillations (QPOs) on year-long timescales, whose physical origin, geometric or plasma-driven, remains debated. In high-synchrotron-peaked (HSP) blazars, departures from a single power-law gamma-ray spectrum, manifested as high-energy upturns in the GeV band, may probe emission mechanisms operating in these sources and their intrinsic duty cycle. Aims. We investigate the link between the 1.7 yr gamma-ray QPO in PKS 2155−304 and an exceptional spectral hardening event identified in the Fermi -LAT population. By mapping this spectral state onto the periodic cycle, we aim to constrain the mechanism driving the oscillation and conditions under which plasma-driven acceleration signatures become visible. Methods. We analyze first 17.4 years of Fermi -LAT data using 30-day binning. To isolate variability on QPO timescales, we apply Singular Spectrum Analysis to mitigate red-noise effects and use a moving block bootstrap approach to quantify correlations between photon flux and photon index while accounting for temporal autocorrelation. We find a statistically significant softer-when-brighter chromatic trend, atypical for HSP blazars and supporting a geometric origin for the flux modulation. Crucially, the spectral hardening event is phase-locked to the QPO trough. This anti-correlation implies that the hardening signature is detectable only when geometrically boosted soft emission is sufficiently suppressed at the flux minimum. Results. We propose a geometric masking scenario in which relativistic jet geometry regulates the visibility of microphysical acceleration processes. The detection of spectral hardening at the flux minimum suggests that such events may be a universal but masked feature of blazar jets, revealed only when Doppler boosting is minimized. These results favor a two-component jet structure and imply that searching for spectral hardening during low-flux states, even in non-periodic sources, may be essential for unveiling the underlying jet physics obscured by relativistic amplification.