return wrong datetime(6) format string

```sql
CREATE TABLE `test` (
  `id` bigint NOT NULL AUTO_INCREMENT,
  `update_time` datetime(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6) ON UPDATE CURRENT_TIMESTAMP(6),
  PRIMARY KEY (`id`)
) ENGINE=InnoDB;
```

the right str:
**update_time: 2026-06-25 12:36:26.575914**

return wrong str:
**update_time: 2026-06-25 12:36:26575914**