Mixed-halide perovskites are promising for high-efficiency multi-junction solar cells but are prone to light-induced halide segregation (Hoke effect), which alters their absorption spectrum and degrades optoelectronic performance. In this study we combined in operando and in situ X-ray scattering methods with complementary optical and electrical measurements to track structural changes, strain evolution, and domain morphology during illumination. Kinetic modelling using the Cahn–Hilliard formalism linked halide migration to observed diffraction peak broadening, indicating formation of Br-rich and Br-poor domains. Even after prolonged relaxation, residual strain and lattice parameter heterogeneity persist, underscoring the complex interplay between illumination, composition, and structural stability in these materials. Our results highlight the complex interplay between illumination, compositional changes, residual strain, and other factors such as humidity, single-crystal quality, or thin-film properties, emphasizing the need to fully understand all influences on the phase segregation effect and to mitigate them for the production of operationally stable devices.