Hi gideon. I am sorry that I did not reply to you sooner.
Above notice was caused by that answer(qa_posts.type=A) is included in SELECT result. Although it might be problem which is possibly dependent only on my environment, I was repaired by adding WHERE clause to SELECT statement.
qa-db-selects.php : qa_db_user_updates_selectspec()
$selectspec['source'].=
" JOIN ^posts AS updateposts ON updateposts.postid=fullevents.lastpostid AND updateposts.type IN ('Q', 'A', 'C')".
" AND (^posts.selchildid=fullevents.lastpostid OR NOT fullevents.updatetype<=>$) AND ^posts.type IN ('Q', 'A', 'C')".
(QA_FINAL_EXTERNAL_USERS ? '' : ' LEFT JOIN ^users AS eventusers ON fullevents.lastuserid=eventusers.userid').
' LEFT JOIN ^userpoints AS eventuserpoints ON fullevents.lastuserid=eventuserpoints.userid'.
" WHERE qa_posts.type='Q'"; // <<<<<<<<<<<<<<<<
I do not have confidence in change ...