Metal halide perovskite interfaces and heterostructures are crucial for engineering future technologies based on these new classes of semiconductors. The structure–function role of the CsPbBr3 and Cs4PbBr6 as mixed phases and their synergistic contribution to emission and efficiency are intensely debated. We show a clear connection between the growth of the competing Cs4PbBr6 phase and the presence of Br vacancies, which serve as the growth nucleation sites. Our understanding is fuelled by a unique cryogenic ultrafast time-resolved cathodoluminescence (TRCL) spectroscopy study of CsPbBr3 and mixed-phase microcrystals. This method precisely pinpoints the spatial location of emission centers and analyzes them spectrally and temporally, unveiling their defect-based origin. Bromide vacancies act as trap states at cryogenic temperatures, resulting in an apparent spectral split easing their detection. We find nonheteroepitaxial growth at the interface of the two phases CsPbBr3/Cs4PbBr6 and agglomeration of precipitants that are bromide-depleted. Our data connects the underpassivated bromide vacancy states at the interface to the enhanced emission from the CsPbBr3/Cs4PbBr6 heterostructures.