Hello PachoChan,
Looking over your description and your screenshots I was able to create a test using your exact steps.
I created a new blank project and duplicated the default floor. I also deleted the fog and added a point light just to add some more light without too much complexity.
I was able to reproduce what you see.
As you can see some areas are rather dark, but at certain angles you can still see some reflection returned. However, right now you have no information that is being returned to your cubemap. Essentially what is happening is that you are forcing the scene to return a reflection value when there is none being calculated. The engine does the best it can within that mode to show you where the reflections would take place.
This is the result after adding a sphere reflection capture.
This is a screenshot from our documentation on sphere and cube reflection captures.
What you are seeing within your project is intended behavior and the above is the step to return accurate reflection values within reflections view mode.
Thank you for reporting this.