Different regions of the Rayleigh-Bénard problem in the parameter space. In the upper part Ra > 0, anti-𝒫𝒯 symmetry is protected, frequencies are purely imaginary, and the system exhibits an instability (black dashed) and no EPs. In the lower part Ra < 0, anti-𝒫𝒯 is no longer protected by anti-Hermiticity, and the system exhibits a curve of EP2s (red) delimiting a region where perturbations propagate. It is a region where anti-𝒫𝒯 symmetry is spontaneously broken.