Well, I find the root cause. In bugnote_api.php

            # only send email if the text is not blank, otherwise, it is just recording of time without a comment.
            if ( $p_send_email && !is_blank( $p_bugnote_text ) ) {
                    email_bugnote_add( $p_bug_id );
            }

$p_send_email is not defined here. After I add $p_send_email = true it works. But I can't understand why?

This is already fixed and will show up in 1.1.3 (see 0009309 for details)