To reproduce: add a RadTextBoxControl and use the following code snippet:

private void radTextBoxControl1_KeyPress(object sender, KeyPressEventArgs e)
{
    if (e.KeyChar == 13)
    {
        radTextBoxControl1.Clear();
    }
}

When you enter some text and hit Enter, the text is cleared, but the caret position remains the same.

Workaround: 

private void radTextBoxControl1_KeyPress(object sender, KeyPressEventArgs e)
{
    if (e.KeyChar == 13)
    {
        radTextBoxControl1.Clear();
        radTextBoxControl1.CaretIndex = 0;
    }
}