To reproduce: use Korean (Microsoft IME) keayboard 
this.radTextBoxControl1.ImeMode = System.Windows.Forms.ImeMode.Hangul;


Workaround:
this.radTextBoxControl1.KeyUp+=radTextBoxControl1_KeyUp;

private void radTextBoxControl1_KeyUp(object sender, KeyEventArgs e)
{
    this.radTextBoxControl1.SelectionStart = this.radTextBoxControl1.Text.Length;
}
Note: the issue can be also observed in the textbox editor in RadGridView.