Is the object visible? It could be just doing a regular draw call from the object's sprite?
Edit: Interesting, I'm not sure why it would show a random sub image. Does the sprite go through all of the indexes?
It's none of business of the ground, Just the cube seems to render on the Z=0?
And you are right, the sub-images of sprite draw step by step. (I scribbles some pattern in the sp_cube) 
This is the one image sprite. And the projection on the ground rotates automatically, although making the rotation of the cube self fixed.
2 years ago(+1)
Try turning off the objects visibility. Fauxton automatically draws models, so the image that is being drawn is more than likely the default drawing done by GM.