Evidence for deviation in gravitational light deflection from general relativity at cosmological scales with KiDS-Legacy and CMB lensing

General relativity (GR) faces challenges from cosmic acceleration and observational tensions, necessitating stringent tests at cosmological scales. In this work, we probe GR deviations via a $\mu$--$\Sigma$ modified gravity parameterization, integrating KiDS-Legacy weak lensing (WL) data (1347 deg$^2$, $z\leq 2.0$), joint cosmic microwave background (CMB) data from Planck, ACT, and SPT, DESI DR2 baryon acoustic oscillation, and DES-Dovekie supernova data. KiDS-Legacy significantly improves constraint precision: $\mu_0$ (matter clustering) by $\sim 60\%$ and $\Sigma_0$ (gravitational light deflection) by $\sim 43\%$ relative to CMB alone. In the $\Lambda$CDM background, $\mu_0 = 0.21\pm 0.21$ is consistent with GR, while $\Sigma_0 = 0.149\pm 0.051$ deviates from GR at the 3.0$\sigma$ level. Furthermore, within the observationally preferred $w_0w_a$CDM background, this deviation in gravitational light deflection persists at the 2.2$\sigma$ level. This deviation is likely driven by the higher amplitudes in the large-scale CMB lensing measurements. This precise separation of GR-consistent matter clustering and deviant light deflection provides key observational clues for new physics or data systematics. Our work underscores the critical role of synergizing high-precision CMB and WL data in advancing GR tests.