ONLY_FULL_GROUP_BY wrong behaviour

When ONLY_FULL_GROUP_BY is set of course you are not allowed to to specify columns in select statement that are not in the group by. But there is an exception: when grouping by id which is a primary key (i.e. unique by definition) all other columns in the same table are functionally dependent upon it. That’s why the following query does not (should not) raise any error: SELECT id, title, genre FROM films GROUP BY id; This acts as expected in MySql, but in mariaDB raises the famous error nonaggregated incompatible with sql_mode=only_full_group_by. Why this happens? There is NOT any possibility to have 2 or more equal records in the same result set since id is a unique primary key. Is this a mariaDB bug or what?