To reproduce:  
            RadTextBox txtAction = new Telerik.WinControls.UI.RadTextBox();
            txtAction.Dock = System.Windows.Forms.DockStyle.Fill;
            this.Controls.Add(txtAction);

Workaround:
void txtAction_Resize(object sender, EventArgs e)
{
            txtAction.TextBoxElement.InvalidateMeasure(true);
            txtAction.TextBoxElement.UpdateLayout();
  }