No keyboard focus on Buttons instantiated from prefabs on canvas,

Currently I am Adding keyboard controlls in order to navigate a Dialog between Player and NPC by WASD and E. I have set up unitys Input controlls for this, but I run into an issue.

When I start the dialog I am able to set the focus to the "next" button (which is not creaated from a prefab) and confirm with "e".

In the next step the Player choices are displayed by instantiating prefab Buttons (see code below)

These buttons do not gain keyboard focus although I set it explictly. I still can click them with the mouse however. I googled and experimented for six hours now but seem unable to find a solution.

maybe some of ou guys has an idea... the Code looks like this:

 void UpdateUI()
         {   
             gameObject.SetActive(playerConversant.IsActive());
  
             if(!playerConversant.IsActive())
             {
                 return;
             }
  
             conversantName.text = playerConversant.GetCurrentConversantName();
             AIResponse.SetActive(!playerConversant.IsChoosing()); 
             choiceRoot.gameObject.SetActive(playerConversant.IsChoosing()); 
  
             if(playerConversant.IsChoosing())
             {
                 BuildChoiceList();
             }
             else 
             {
                 AIText.SetText(playerConversant.GetText());
                 nextButton.gameObject.SetActive(playerConversant.HasNext());
                 //put focus on the button...this works
                 EventSystem.current.SetSelectedGameObject(nextButton.gameObject);
             }   
         }
  
         private void BuildChoiceList()
         {
           
             foreach (Transform item in choiceRoot)
             {
                 Destroy(item.gameObject);
             }
  
             foreach (DialogNode choice in playerConversant.GetChoices()) 
             {
                 GameObject choiceInstan = Instantiate(choicePrefab, choiceRoot);
  
                 choiceInstan.GetComponentsInChildren<TextMeshProUGUI>().FirstOrDefault().SetText(choice.GetText());
  
                 choiceInstan.GetComponentsInChildren<Button>().FirstOrDefault().onClick.AddListener(() =>
                 {
                     playerConversant.SelectChoice(choice);
  
                 });
             }
             // Try to set fokus to first instantiated button
             Button toSetFocusOn = choiceRoot.GetComponentsInChildren<Button>().FirstOrDefault();
             toSetFocusOn.gameObject.SetActive(true);
             EventSystem.current.SetSelectedGameObject(toSetFocusOn.gameObject);   
         }
     }

The hirarchy setup looks lke this