set group_concat_max_len=64;

create table t30 (a varchar(254));

insert into t30 values (concat('x64-', repeat('a', 60)));

insert into t30 values (concat('x64-', repeat('b', 60)));                                                                                              

insert into t30 values (concat('x64-', repeat('c', 60)));

MariaDB [test]> select json_arrayagg(a) from t30;                                                                                                                      

+--------------------------------------------------------------------+                                                                                                 

| json_arrayagg(a)                                                   |                                                                                                 

+--------------------------------------------------------------------+                                                                                                 

| ["x64-aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa] |

+--------------------------------------------------------------------+

1 row in set, 1 warning (0.002 sec)

MariaDB [test]> show warnings;

+---------+------+---------------------------------+

| Level   | Code | Message                         |

+---------+------+---------------------------------+

| Warning | 1260 | Row 1 was cut by GROUP_CONCAT() |

+---------+------+---------------------------------+