The selection of related tags is comparing incorrectly for access level:

WHERE tag_id != $1 AND bug_id IN ( SELECT b.id FROM mantis_bug_table b

LEFT JOIN mantis_project_user_list_table p

ON p.project_id=b.project_id AND p.user_id=$2 JOIN mantis_user_table u

ON u.id=$3 JOIN mantis_bug_tag_table t

ON t.bug_id=b.id

WHERE ( p.access_level>b.view_state OR u.access_level>b.view_state )

AND t.tag_id=$4 )

The clause
WHERE ( p.access_level>b.view_state OR u.access_level>b.view_state )
is comparing to view_state which is not an access level.