This query almost certainly throws a warning about the data being cut by
GROUP_CONCAT. If you are using a client or library that ignores warnings, you can run the query
SHOW WARNINGS; immediately after running your query, and confirm that this warning is present.
To resolve this, you need to increase the value of the system variable
@@GROUP_CONCAT_MAX_LEN to get longer results without truncation.
The default value is only 1024 (bytes, not characters, according to the documentation). You can change this in global config or change it in your session before running this query.