How do I stop the 3D overlap box around objects?

C3/Mobile game

Is there a way to stop the odd box that appears around some sprites in the 3D games?

When the game is running and an object overlaps another object, part of the overlapped object either changes color or vanishes until the original object moves out of the way.

Changing the collision points doesn't seem to have an effect.

Thoughts?

See the section on transparency in Using 3D in Construct.