Thanks for the images, but scene files are the way to go. Images are so limited, especially when they crop out pretty much all of the interface.
What I can see is that it is not a problem with an axis. You use them only to describe what you want and what you don’t want, while in this case, the deciding coordinate values are + or -.
I don’t have any hints about why the cubes are in the scene. With a scene file, I might get a clue, etc.
In the past nearly 17 years of helping/mentoring people, I have certainly concluded that guessing is a waste of everyone’s time. But with scene files, you have something that works, or it doesn’t. So, having said that, perhaps you like to use the Boole-tool?
If you like to have that Y value as the deciding parameter, and you can’t look underneath of the objects, the Formula Shader will work when placed in the Alpha channel, formula: ?(y>0;1;0)
Which reads: if y is greater than zero, the shader-value will be one, otherwise zero. In this way, the object will be visible above y-zero. In this case, the short formula y>0 would work as well.
Scene files (01 Boole, 11 formula)
All the best