Isso cheira a um inseto. Se você precisar contorná-lo e implementar sua lógica, alternativamente, você pode usar a fatoração de subconsulta recursiva (recursiva com), que funciona bem na versão 11.2.0.4:
SQL> with t (id, label, parentid, reportlevel, fake_connect_by_path) as (
2 select id, label, parentid, 0 as reportlevel, ' -> ' || label as fake_connect_by_path
3 from temptable
4 where parentid is null
5 union all
6 select tt.id, tt.label, tt.parentid, reportlevel + 1, t.fake_connect_by_path || ' -> ' || tt.label as fake_connect_by_path
7 from temptable tt
8 join t on t.id = tt.parentid
9 )
10 select fake_connect_by_path
11 from t;
FAKE_CONNECT_BY_PATH
--------------------------------------------------------------------------------
-> ninechars
-> Im stumped
-> - Unknown -
-> ninechars -> erewrettt
