ONLY_FULL_GROUP_BY not restricting ORDER BY

ONLY_FULL_GROUP_BY does not appear to be restricting ORDER BY when I would have thought it would. For example, in the following, the ORDER BY specifies a non-grouped column leading to non-determinate results:

create table documentdb (timestamp datetime, filename text) select now() timestamp, 'foo' filename union all select now() - interval 2 minute, 'foo' union all select now() - interval 1 minute, 'bar';

set session sql_mode=concat('only_full_group_by,',@@sql_mode);

select filename from documentdb group by filename order by timestamp desc limit 10;

produces

foo

bar

mysql 8 gives an error, as I would have expected mariadb to do: "Expression #1 of ORDER BY clause is not in GROUP BY clause and contains nonaggregated column 'db_116865070.documentdb.timestamp' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by"