When lumen is attempting to draw a reflection, first it searches for a screen trace (essentially an improved SSR). The floating cube blocks the screen space view of the objects in the horizon, so it must fall back to lumens raytraced reflections instead of a screen trace for drawing the reflection in the water. How this will look depends on your settings, but usually Hardware RT with “hit lighting” as you have set offers the best results.
However, not all material features are currently supported for hit lighting. Go into the Lumen overview viewmode and I suspect that many of the elements in the scene will be black.
You can use the node “RaytraceQualitySwitch” to make the material disable unsupported features in reflections. For example, metallic objects appear black under lumen hit lighting. The quality switch would allow the object to be metal when viewed normally, but non-metallic when viewed in a reflection, this preventing it from turning black.
You’ll also need to consider tracing distance. A ray will only travel so far for performance reasons. The exact specifications depend on settings. Take a look at the following link and pay special attention to the section labeled far field tracing.
Lumen Technical Details in Unreal Engine | Unreal Engine Documentation.