PHP $ conn-> closed（）不在PROCESSLIST表中删除进程

I have AWS RDS - MariaDB 10.1.31, db.t2.medium. and I always use this code to connect and close into RDS instance :

$conn = new mysqli($servername, $username, $password, $dbname);

//--- SOME CODES HERE

$conn->close();

I run my codes using cron every minute. Then I checked the process using this command :

mysql> SELECT IP_ADDRESS, COUNT(*) AS NUM FROM ( SELECT SUBSTRING(HOST, 1, CHAR_LENGTH(HOST) - 6) AS IP_ADDRESS FROM INFORMATION_SCHEMA.PROCESSLIST  GROUP BY HOST ) AS t1 GROUP BY IP_ADDRESS ORDER BY IP_ADDRESS DESC;
+--------------+-----+
| IP_ADDRESS   | NUM |
+--------------+-----+
| localhost    |   1 |
| 172.31.9.xxx |   4 |
| 172.31.3.yy  |  10 |
| 172.31.2.zzz |   1 |
+--------------+-----+
4 rows in set (0.34 sec)

I run that command every second and realise that IP address from one of EC2 instance still there even I put $conn->close(); in my codes.

it seems that the process stuck in the PROCESSLIST table.

how to immediately have PROCESSLIST table clear after $conn->close(); executed?

thank you.