This letter discusses the temperature dependence of the magnetic field at which irreversible magnetic flux loss of ferrite sintered magnets is initiated. In this material class such a field might reduce linearly: comparable behavior to that observed for Nd-Fe-B-based sintered counterparts increases linearly or even peaks for temperatures above 350 K. These differences are attributed to variations in the saturation polarization and magnetocrystalline anisotropy constant(s) of the hard-magnetic phase arising from changes in the chemical composition. A comparison between HK and HD5, where the latter is defined in an IEC standard, is also presented and discussed.