Select with group by behavior in 5.5 vs 10.0

Hi,

I've upgraded from MariaDB 5.5.27 to 10.0.11 and noticed that adding a group by to the query below now returns all NULL values for the third column (b.oid) whereas in 5.5.27 it returns the matching row in table b. I realize this is non-standard SQL since I'm selecting a non aggregated column that is not in the group by, but I would expect that it would find the one matching row in table b as it did in 5.5.27. Is it by design that it behaves this way now in 10.0 or is this a bug?

Query without a group by clause:

MariaDB [group_by_test]> select a.oidGroup, a.oid, b.oid from table_one a left join table_two b on a.oid=b.oid where a.oidGroup=1;
+----------+-----+------+
| oidGroup | oid | oid  |
+----------+-----+------+
|        1 |   1 | NULL |
|        1 |   2 | NULL |
|        1 |   3 |    3 |
|        1 |   4 | NULL |
+----------+-----+------+
4 rows in set (0.00 sec)

With group by in v5.5.27 (same results):

MariaDB [group_by_test]> select a.oidGroup, a.oid, b.oid from table_one a left join table_two b on a.oid=b.oid where a.oidGroup=1 group by a.oid;
+----------+-----+------+
| oidGroup | oid | oid  |
+----------+-----+------+
|        1 |   1 | NULL |
|        1 |   2 | NULL |
|        1 |   3 |    3 |
|        1 |   4 | NULL |
+----------+-----+------+
4 rows in set (0.00 sec)

With group by in v10.0.11 (all NULL values for column 3 as if there were no matching rows):

MariaDB [group_by_test]> select a.oidGroup, a.oid, b.oid from table_one a left join table_two b on a.oid=b.oid where a.oidGroup=1 group by a.oid;
+----------+-----+------+
| oidGroup | oid | oid  |
+----------+-----+------+
|        1 |   1 | NULL |
|        1 |   2 | NULL |
|        1 |   3 | NULL |
|        1 |   4 | NULL |
+----------+-----+------+
4 rows in set (0.05 sec)

Thanks in advance!