Pressing Tab/Shift+Tab when caret position is at the beginning of a paragraph should change current paragraph FirstLineIndent instead of LeftIndent.

First line indent should be changed to Document.DefaultTabWidth step.

Workaround:
private void RadRichTextEditor1_KeyDown(object sender, KeyEventArgs e)
{
    if (e.KeyData != Keys.Tab)
    {
        return;
    }

    this.radRichTextEditor1.Insert("\t");
    e.SuppressKeyPress = true;
    e.Handled = true;
}