These highlight-protection modes work by biasing exposure to protect bright areas from clipping. In practice, the camera exposes a bit darker than normal, then adjusts the image processing so midtones and highlights look more natural afterward.

That helps preserve highlight detail, especially in bright scenes, but it comes with a tradeoff: shadows can end up darker and noisier because the image is effectively underexposed.

The likely reason ISO expansion is disabled is that the camera is already altering its effective ISO/exposure behavior internally to reduce overall exposure for highlight protection. ISO expansion also changes the available ISO range or how the camera handles gain, so the two features can conflict. In short, both settings affect exposure/gain strategy, and the camera prevents you from combining them because highlight-priority mode depends on controlling that process itself.