One of the new features in 3D Element v.2 are real-time reflections of the elements of the scene, not just the enviroment of the v.1. We have two new types, selectables from the object options within the scene editor: mirror surface and spherical.
As you guess, mirror surface is perfect for flat surfaces like floors, walls and mirrors and spherical to spherical objects or with an irregular geometry. The cube does not fit with either ways. We can see it in the picture below, where the floor uses mirror surface and spheres and cube, spherical. Notice that the reflected image by the cube is not what we expect.
The solution is to dissect the cube in what in fact is, a rig of 6 planes. Reposite and rotate the 6 planes to create a compact cube, assigning each one mirror surface reflection mode .
We can see in the picture below that the resulting reflection is closer to what we expected. It’s a slow solution and we lose certain properties of the cube primitive, but E3D has certain technical limitations arising from its conception in real time which we must deal with.