C# can't update ComboBox when Item List is filled

Question

I have a function which clears all Comboboxes on the form and then try to update the text. The new text is only shown if no Items are in the Item list. Does anyone have an idea whats wrong?

I deleted all functions from the project so that only the necessary part is available.

https://www.dropbox.com/s/ibst7enrteyk9jb/Digitales_Auftragsformular.zip?dl=0

The project is attached. Simply start, go to tab "Werkzeuganfrage" there are two Comboboxes in red. One without Items = working, one with Items which is not working.

Answer 1

 public Oberflaeche()
    {
        InitializeComponent();           


        List<ComboBox> myComboBoxes = GetControlsByType<ComboBox>(this, typeof(ComboBox));

        foreach (ComboBox txt in myComboBoxes)
        {
            //txt.Text = "";
            txt.SelectedIndex = -1;

        }
        Werkzeuganfrage_Combobox_rhino1_1.Text = "Rhino1_1";
        Werkzeuganfrage_Combobox_rhino1_2.Text = "Rhino1_2";

        comboBox1.Text = "Rhino1_1";
        comboBox2.Text = "Rhino1_2";
    }



    public List<T> GetControlsByType<T>(Control container, Type type)
    {


        List<T> result = new List<T>();
        foreach (Control c in container.Controls)
        {
            if (c.GetType() == type)
            {
                result.Add((T)Convert.ChangeType(c, typeof(T)));
            }

            if (c.HasChildren)
            {
                result.AddRange(GetControlsByType<T>(c, type));
            }
        }
        return result;
    }