To reproduce: Using the below code, type something in the text box and execute the button shortcut => the caret remains where it was

 protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);

            AddTextBoxControl();

            radButton1.ButtonElement.Shortcuts.Add(new RadShortcut(Keys.Alt, new Keys[] { Keys.R }));
        }

        private void radButton1_Click(object sender, EventArgs e)
        {
            radTextBoxControl1.Text = "";
        }

Workaround:
Instead of setting the Text to empty string, use the control Clear method.