Observed polarization degree (upper panels) and flux (lower panels) at observer frequency ν = 10¹⁵ Hz (PLS G) with a flat distribution of initial Lorentz factor (b = 0) and a random magnetic field structure confined to the face of the shock (ξ → 0) for off-axis jets. The direction of the polarization vector remains constant throughout the temporal evolution. Left panel: The viewing angle is set to be constant for an off-axis jet with q = 5 with varying values of a. The polarization degree reduces with a and its peak coincides with a light curve break. We mark with a star and vertical dashed lines the time of a light curve dip, expected when the emission transitions from angular structure dominated to core dominated in the Blandford-Mckee deceleration regime (Beniamini et al. 2020). At this time, the onset of a plateau phase can be seen in the polarization curve. Right panel: The jet structure is held constant with a = 3.5 while the off-axis viewing angle changes. As the observer line of sight approaches the jet symmetry axis with reducing values of q, the polarization peak becomes lower and occurs at earlier times.