How vulkan process missing component of swizzle?

The OpenGL 4.6 Spec said:

but i don’t find any description about missing component in vulkan. For example, i use texture() function to sample a image with the format of r8_unorm and the swizzle of xyzw. The texture() function should return (r, 0, 0, 0) or (r, 0, 0, 1). Although i test it on amd, and get (r, 0, 0, 1), but i want to know whether there is specific description about this in spec.

It’s in the chapter titled “Conversion to RGBA”.