I spent some time trying to solve a problem that I’m experiencing here, I activate the transparent texture, I put the transparent PNG but when I test it, there’s a gray background where it should be transparent.
The part of the image below that is marked in white you can see the error
The problem will be because the z order of the 3D objects is the same or lower than the floor object, and the back of the box cuts out the floor. The grey you are seeing will be your background scene colour.
So you can either
Separate the 2 layers, so the floor is on a 2D layer below the 3D layer, or
Change the z-order of the floor so it is less than or below that of the 3D objects.
Both these will resolve the issue:
Problem - 2D & 3D mixed layer, same z-order (green background):
Solution 1 - 2D and 3D separate layers:
Solution 2 - 2D and 3D mixed layer, 3D box z-order greater than the floor z-order: