我无法重现该问题。
测试:
mysql> SELECT VERSION();
+-----------+
| VERSION() |
+-----------+
| 5.7.11 |
+-----------+
1 row in set (0.00 sec)
mysql> SET @`test` := '{"foo": {"efg":1}, "bar": {"abc":0}}';
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT JSON_INSERT(@`test`,/*'$.foo', 10,*/ '$.foo.klm', 2);
+--------------------------------------------------+
| JSON_INSERT(@`test`, '$.foo.klm', 2) |
+--------------------------------------------------+
| {"bar": {"abc": 0}, "foo": {"efg": 1, "klm": 2}} |
+--------------------------------------------------+
1 row in set (0.00 sec)
更新
mysql> DROP TABLE IF EXISTS `table1`;
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> CREATE TABLE IF NOT EXISTS `table1` (
-> `id` INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
-> `test` JSON
-> );
Query OK, 0 rows affected (0.00 sec)
mysql> INSERT INTO `table1`
-> (`test`)
-> VALUES
-> ('{"foo": {"efg":1}, "bar": {"abc":0}}');
Query OK, 1 row affected (0.01 sec)
mysql> SELECT `id`, `test` FROM `table1`;
+----+----------------------------------------+
| id | test |
+----+----------------------------------------+
| 1 | {"bar": {"abc": 0}, "foo": {"efg": 1}} |
+----+----------------------------------------+
1 row in set (0.00 sec)
mysql> SELECT JSON_INSERT(@`test`, '$.foo.klm', 2)
-> FROM `table1`
-> WHERE `id` = 1;
+--------------------------------------------------+
| JSON_INSERT(@`test`, '$.foo.klm', 2) |
+--------------------------------------------------+
| {"bar": {"abc": 0}, "foo": {"efg": 1, "klm": 2}} |
+--------------------------------------------------+
1 row in set (0.00 sec)
mysql> UPDATE `table1`
-> SET `test` = JSON_INSERT(@`test`, '$.foo.klm', 2)
-> WHERE `id` = 1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> SELECT `id`, `test` FROM `table1`;
+----+--------------------------------------------------+
| id | test |
+----+--------------------------------------------------+
| 1 | {"bar": {"abc": 0}, "foo": {"efg": 1, "klm": 2}} |
+----+--------------------------------------------------+
1 row in set (0.00 sec)