Your problem is not with parameter User Search Pattern, but with Search Base DN. Your Search Base DN should point to LDAP tree where can find all your users/groups you use in your ldap filter.
So try to change your Seach Base DN to:
OU=CUSTOM,DC=SUBDOMAIN,DC=DOMAIN,DC=NET
And I should work as expected with your original User Search Pattern.
In my setup I use little advanced LDAP filter:
(&(objectCategory=person)(objectClass=user)(sAMAccountName={0})(memberOf:1.2.840.113556.1.4.1941:=CN=Graylog_users,CN=Users,DC=domain,DC=com)(!(userAccountControl:1.2.840.113556.1.4.803:=2)))
This is recommended way to search users, because it is cached and faster as you’ve already included:
(objectCategory=person)(objectClass=user)
This parameter allows add to Graylog_users group also another group (not only users):
memberOf:1.2.840.113556.1.4.1941:=CN=Graylog_users,CN=Users,DC=domain,DC=com
This disallow blocked users:
!(userAccountControl:1.2.840.113556.1.4.803:=2)
Hope this helps.