Contains(Input.mousePosition) reacts to wrong rect

In my code I have 12 buttons. All with their own functionality and all work as intended. For six of them I added a "MouseOver" functionality and I now want to do that for the other six too.

But I am running in a weird problem.

Definition of the buttons:

  private Rect ButtonO1;
         private Rect ButtonO2;
         private Rect ButtonO3;
         private Rect ButtonU1;
         private Rect ButtonU2;
         private Rect ButtonU3;
         private Rect ButtonLO1;
         private Rect ButtonRO1;
     
     void Start () {
             ButtonO1 = new Rect(Screen.width/2+70,Screen.height/5-55,50,50);
             ButtonO2 = new Rect(Screen.width/2-25,Screen.height/5-55,50,50);
             ButtonO3 = new Rect(Screen.width/2-120,Screen.height/5-55,50,50);
             ButtonU1 = new Rect(Screen.width/2+70,Screen.height-90,50,50);
             ButtonU2 = new Rect(Screen.width/2-25,Screen.height-90,50,50);
             ButtonU3 = new Rect(Screen.width/2-120,Screen.height-90,50,50);
             
             ButtonLO1 = new Rect(Screen.width/2-200,Screen.height/5+25,50,50);
             ButtonRO1 = new Rect(Screen.width/2+150,Screen.height/5+25,50,50);
         }

In the Update function I have this

 void Update () {
         if         (ButtonO1.Contains(Input.mousePosition) || ButtonU1.Contains(Input.mousePosition)) {
             OnMouseOverO1();}        
         else if (ButtonO2.Contains(Input.mousePosition) || ButtonU2.Contains(Input.mousePosition)) {
             OnMouseOverO2();}        
         else if (ButtonO3.Contains(Input.mousePosition) || ButtonU3.Contains(Input.mousePosition)) {
             OnMouseOverO3();}
         else if (ButtonLO1.Contains(Input.mousePosition) || ButtonRO1.Contains(Input.mousePosition)) {
             Debug.Log("Button L1 oder R1");}
         else {
             OnMouseExit();}        
     }

The first six buttons work flawlessly. When I hover over them it does what it is supposed to do. If I hover over the ButtonLO1 or RO1 nothing happens. The Debug.Log command is only processed when I hover over two of the remaining four buttons. And I have no idea why!!! If I press the ButtonLO1 it works. So some part of the engine realizes I am hovering and clicking that button even so the Contains(Input.mousePosition) clearly doesn't.

Does anyone know why the engine confuses those buttons? Why does it nothing happen if I hover over ButtonLO1? Should I report a bug?