Take a look at the game Tharsis (2016) created in unity engine:
Notice how the dice rolls in this game are a separate 3D object rendered on the UI with the space station rendered normally in the background. The dice rolls have their own lighting and cast shadows from their own point light. The dice are not falling on top of any visible platform (invisible floor), yet the dice cast shadows onto this invisible floor (see video) and the shadows darken the scene behind them.
I’m trying to find out how to achieve this in unreal.