Why is my y-axis bugging?

Hi, all!

I'm currently working on an assignment from my university and can't figure this following thing out.

In my project, I have a sphere that follows my mouse. When I touch a cube, it's selected and I want to perform transformations on it. When I hold the left mouse click, I want to move the cube.

The following script comes from the update() in my sphere:

             Vector3 spherePosition = transform.position;
 
             // Modify position
             if (Input.GetMouseButton(0))
             {
                 Debug.Log("Current Sphere position: " + spherePosition);
                 selected[0].transform.position = spherePosition;
             }

The "selected" is a Cube-element.

Now the issue is that, although I'm not moving the mouse, the y-axis is acting weird every other frame. I jumps to much lower values so that I sometimes loose it in the camera fram when I let go from the left mouse click:

I could not help myself with googling so thanks in advance for any tips!