Light soaking impacts perovskite solar cells, causing cation rotation, octahedral distortion, and weakened hydrogen bonding. Using a unique in-operando setup for ISOS protocols, we monitor structural, optical, and electrical responses under prolonged light exposure, revealing progressive average changes without sample reloading uncertainties. Over 20 h intervals, light-induced lattice deformation causes progressive local demixing, partially reversible in dark, and residual amorphization that hinders electrical recovery. Lattice expansion and bandgap red-shift indicate increasing iodide local enrichment, while a bandgap blue-shift occurs under heating. FA-MA-Cs-perovskites resist to this ionic demixing more than FA-Cs. Sunlight is the primary trigger for that, surpassing the effects of bias or induced heating. Stress tests at 65 °C drive both formulations from demixing to irreversible phase segregation, with FA-Cs devices showing greater structural and electrical resilience than FA-MA-Cs. Since a demixing–remixing interplay governs the device operation, we recommend tracking it using in-operando protocols over 24–48 h of unaccelerated sunlight–dark testing.