As far as i can see, they are fully reflected. However, since their lower part is black, only black is reflected. An usual spherical HDRI has color for the ground too, but both of your shown HDRI seem to be simple panoramas with a completely black lower half. So een if you deactivate “Lower half is solid color”, then you will still see black, because the actual HDRI is black in that region. For a test, you could take one of those HDRI and color their lower half red or green, and see, if that gets reflected or not.
Or grab one from here: HDRIs: Skies • Poly Haven
That being said, in your chromeball, this half dark-half-bright reflection from the upper half of the hdri is interesting. This shouldn´t be there, it should be uniform, judging by the displayed hdri. Try to go into the texture and change it´s texture group from skybox to world, and see, if that does something helpful. Or just make an entirely new project, and test it there, if this reflection still create this weird reflection.
Also, to get a normal visible Sky, use the HDRI backdrop, that is there for this exact reason (if you don´t like the sphere solution). And check the option “Camera projection” to get rid of the distorted ground, or change its mesh to “Sphere_inversenormals” (it´s a sphere with inverted normals, check this thread: https://forums.unrealengine.com/deve…11#post1808511 ).
A skylight is not there to create a sky,** that is visible for the camera**. You need a geometry with your HDRI applied as material, to make your sky visible… and that is, what the HDRI backdrop is, or any mesh with a hdri cubemap applied.