Create a new blank project, new blank map, enable forward rendering. Go to reflection view mode and place a sphere reflection capture, you will see the default sky sphere in the reflection on the floor. Place a stationary or dynamic sky light, and the reflections will get brighter, until you update or move the reflection capture, then the entire sky reflection will go black. Add some static meshes to the level that can be seen in the reflection capture and you’ll notice that the top half of the sky is actually being additively mixed on top of the reflection environment without regards to depth, and the place where the sky should show through is pure black. Go to the sky light and uncheck Lower Hemisphere Is Solid Color and you will see that the sky below the horizon is now also adding on top of the reflection environment.
This bug does not appear with static sky lights.