Updating a row that has a TTL produce unexpected results

aboudreault@kovarro:~/dev/cstar/so27280407$ ./test.sh 
Current cluster is now: local
Insert data with a 5 sec TTL
INSERT INTO ks.tbl (pk, foo, bar) values (1, 1, 'test') using TTL 5;

 pk | bar  | foo
----+------+-----
  1 | test |   1

(1 rows)

Update data with no TTL
UPDATE ks.tbl set bar='change' where pk=1;
sleep 6 sec
BUG: Row should be deleted now, but isn't. and foo column has been deleted???

 pk | bar    | foo
----+--------+------
  1 | change | null

(1 rows)

Insert data with a 5 sec TTL
INSERT INTO ks.tbl (pk, foo, bar) values (1, 1, 'test') using TTL 5;

 pk | bar  | foo
----+------+-----
  1 | test |   1

(1 rows)

Update data with a higher (10) TTL
UPDATE ks.tbl USING TTL 10 set bar='change' where pk=1;
sleep 6 sec
BUG: foo column has been deleted?

 pk | bar    | foo
----+--------+------
  1 | change | null

(1 rows)

sleep 5 sec
Data is deleted now after the second TTL set during the update. Is this a bug or the expected behavior?

(0 rows)