Ion migration and phase segregation continue to limit the application of mixed halide hybrid perovskite materials in (opto)electronic devices. To better understand the underlying causes behind these effects, photoluminescence (PL) spectroscopy is utilized to study a series of mixed-halide hybrid perovskites with the general formula MAPb(I_0.4Br_0.6)_x, focusing on subtle alterations to the perovskite's halide stoichiometry (2.94 ≤ x ≤ 3.04). These findings reveal that even slight variations in halide stoichiometry and/or illumination conditions significantly affect the rate and extent of photo-induced phase segregation in complex ways, which subsequently influences important optoelectronic properties, in particular, changes to the luminescence properties of the material. It is demonstrated that the average excitation power and frequency strongly influence the segregation rate, independently of stoichiometric composition. These findings demonstrate the high sensitivity of these material systems to small changes in manufacturing conditions, which improves the general understanding of this phenomenon and will aid in the development of advanced fabrication techniques aimed at minimizing light-induced phase segregation in hybrid perovskite electronic and optoelectronic devices.