undocumented feature or bug in subquery : psql (PostgreSQL) 9.4.6 on SLES12 SP1

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.4/static/functions-subquery.html
Description:

psql (PostgreSQL) 9.4.6 on SLES12 SP1
2 tables a and b
               Table "public.a"
     Column      |           Type           |   Modifiers
-----------------+--------------------------+---------------
 foo             | integer                  | not null
 bar             | integer                  | not null

               Table "public.b"
     Column      |           Type           |   Modifiers
-----------------+--------------------------+---------------
 bar             | integer                  | not null

No error when executing this statement even though table b does not contain
column foo
SELECT foo,bar FROM a WHERE foo=1 AND bar NOT IN (SELECT bar FROM b WHERE
bar > 2 AND foo=2)

Get an error on this one ERROR:  column b.foo does not exist
SELECT foo,bar FROM a WHERE foo=1 AND bar NOT IN (SELECT bar FROM b WHERE
b.bar > 2 AND b.foo=2)

It works if I remove foo from the subquery.
SELECT foo,bar FROM a WHERE foo=1 AND bar NOT IN (SELECT bar FROM b WHERE
b.bar > 2)

Do I need to qualify all columns in the subquery or is this a bug is psql
9.4.6?