SQL Developer suggesting I add aggregate to group by clause
Looks like a bug in (the parser of) the specific version of SQL Developer (4.1.5.21) you use.

Update to the most recent version and if it shows the same behaviour, send a bug report to Oracle.

You could try to find a smaller example (something like the below, if it does exhibit the same buggy suggestions):

CREATE TABLE t
  ( a INT NOT NULL, 
    b DATE NOT NULL
  ) ;

-- no suggestions
SELECT /*+ materialize */ a, MIN(b) AS min_b
FROM t
GROUP BY a ;

-- buggy suggestions
WITH ct AS
  ( SELECT /*+ materialize */ a, MIN(b) AS min_b
    FROM t
    GROUP BY a 
  ) 
SELECT * 
FROM ct ;