The firstButton.Focus() is currently called in the OnEnable() method. That does not work because the focused element needs to be provided in the Start() method. Otherwise the EventManager initialization is done, without the event manager knowing the first selected element. So the best practice at the moment is to use the .Focus() method in Start(). I am referring to the following documentation: https://docs.unity3d.com/Manual/UIE-faq-event-and-input-system.html