This isn't entirely random. When Unicode started including colourful emoji rather than just monochrome glyphs they added an attribute to each character to say w

This isn't entirely random. When Unicode started including colourful emoji rather than just monochrome glyphs they added an attribute to each character to say whether it should be rendered in colour by default. And they retrospectively gave a bunch of existing symbols the colourful attribute. As you noted, this broke a bunch of existing use cases. It is infuriating.