Reproducing

1. Create two simple tables and fill them with rows:

   CREATE TABLE `table1` (
       `id` INT AUTO_INCREMENT,
       `link` INT,
       `field1` INT,
       PRIMARY KEY (`id`)
   )
   ENGINE=InnoDB;
   
   CREATE TABLE `table2` (
       `id` INT AUTO_INCREMENT,
       `link` INT,
       `field2` INT,
       PRIMARY KEY (`id`)
   )
   ENGINE=InnoDB;
   
   INSERT INTO `table1` (`link`, `field1`) VALUES (3, 1);
   INSERT INTO `table1` (`link`, `field1`) VALUES (1, 2);
   INSERT INTO `table1` (`link`, `field1`) VALUES (2, 3);
   
   INSERT INTO `table2` (`link`, `field2`) VALUES (2, 1);
   INSERT INTO `table2` (`link`, `field2`) VALUES (3, 1);
   

2. (optional) Enable outer_join_with_cache optimization, if it's disabled by default:

   SET SESSION optimizer_switch='outer_join_with_cache=on';
   

3. Select all rows, joining by `link`, ordering by `field1`, using group_concat function in select query:

   select
       field1, t1.link, field2, group_concat('any string or field')
   from table1 t1
       left join table2 t2 on t1.link = t2.link
   group by field1
   

   Notice everything works fine now:

   field1	link	field2	group_concat
   1	3	1	any string of field
   2	1	NULL	any string of field
   3	2	1	any string of field

4. Disable outer_join_with_cache optimization:

   SET SESSION optimizer_switch='outer_join_with_cache=off';
   

5. Repeat step 3.
   Notice that the result of the query has been changed: there is NULL value instead of 1 - it's wrong:

   field1	link	field2	group_concat
   1	3	NULL	any string of field
   2	1	NULL	any string of field
   3	2	1	any string of field

6. If your database is small you can control this behavior by switching outer_join_with_cache on and off, otherwise it seems like outer_join_with_cache is always turned off, so it always reproduces on big tables, whether outer_join_with_cache is on or off.

Other points

1. Using group_concat seems to be a necessary condition for reproducing this bug.
2. If you will order the query in step 3 by `t1.link`:

   select
       field1, t1.link, field2, group_concat('any string or field')
   from table1 t1
       left join table2 t2 on t1.link = t2.link
   group by field1
   order by t1.link
   

   everything will work fine:

   field1	link	field2	group_concat
   2	1	NULL	any string of field
   3	2	1	any string of field
   1	3	1	any string of field