Tooltip not displayed correctly in inspector

I want to use the Tooltip for my script attributes to display additional information when they get hovered by the mouse. My problem is, it appears even the mouse didn't hover the attribute inside the inspector.

I set it up like this:

 [Tooltip("blabla")]
 public ButtonLineRenderer _buttonLineRenderer;

The result is:

I searched for this problem but everybody tells just adding the Tooltip above the attribute in the script. I am using a Monobehaviour and also don't have any errors. Can anybody help me with this problem?