Default text in one text-box disappear when click on another text-box

See more:

TextBoxBase

I have text-boxes and their texts are set by default to normal or none, the problem is that when I click on one text-box and then click on another text-box, all the other text-boxes default text will disappear.

You can check this picture to understand my problem:
Imgur: The magic of the Internet[^]
[^]

What I have tried:

The code where I set the text-boxes default text

private void AddButton_Click(object sender, EventArgs e)
   {

       objState = EntityState.Added;

       metroPanel2.Enabled = true;


       eRCPclassBindingSource.Add(new ERCPclass());
       eRCPclassBindingSource.MoveLast();


       PatientNamebox.Focus();
       PatientNamebox.Text = null;
       Agebox.Text = null;
       refbox.Text = null;
       MidazolamBox.Text = "None";
       PethidineBox.Text = "None";
       OtherDuruBox.Text = "None";
       PapillaBox.Text = "None";
       CholangiographyBox.Text = "Normal";
       PancreatographyBox.Text = "Normal";
       TherapeuticProceduresBox.Text = "Normal";
       ConclusionBox.Text = "Normal";
       RecommendationBox.Text = "Normal";
       pic.Image = null;
       pic1.Image = null;
       pic2.Image = null;
       pic3.Image = null;
       pic4.Image = null;
       pic5.Image = null;

       label24.Text = dgvERCPinfo.RowCount.ToString();

   }