event_scheduler acquires Backup lock and does not release upon completion

mysql [localhost:10504] {msandbox} (test) > create table emp (id int, name varchar(50), hiredate date, primary key(id));

Query OK, 0 rows affected (0.015 sec)

mysql [localhost:10504] {msandbox} (test) > create table emp_view (id int, name varchar(50), hiredate date, primary key(id));

Query OK, 0 rows affected (0.007 sec)

mysql [localhost:10504] {msandbox} (test) > insert into emp values(1, 'Muhammad', CURDATE());

Query OK, 1 row affected (0.005 sec)

mysql [localhost:10504] {msandbox} (test) > 

mysql [localhost:10504] {msandbox} (test) > delimiter //

mysql [localhost:10504] {msandbox} (test) > create procedure update_emp_report()

    -> BEGIN

    ->   TRUNCATE TABLE `emp_view`;

    ->   INSERT INTO `emp_view` SELECT * FROM emp;

    -> END

    -> //

Query OK, 0 rows affected (0.003 sec)

mysql [localhost:10504] {msandbox} (test) > delimiter ;

mysql [localhost:10504] {msandbox} (test) > 

mysql [localhost:10504] {msandbox} (test) > CREATE EVENT `update_emp_report_every_2_minutes` ON SCHEDULE EVERY 2 MINUTE STARTS '2020-10-01 00:00:00' ON COMPLETION NOT PRESERVE ENABLE DO CALL update_emp_report();

Query OK, 0 rows affected, 1 warning (0.001 sec)

mysql [localhost:10504] {msandbox} (test) > show tables;

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

| Tables_in_test |

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

| emp            |

| emp_view       |

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

2 rows in set (0.000 sec)

mysql [localhost:10504] {msandbox} ((none)) > select * from  information_schema.metadata_lock_info\G

*************************** 1. row ***************************

    THREAD_ID: 4

    LOCK_MODE: MDL_BACKUP_SYS_DML

LOCK_DURATION: NULL

    LOCK_TYPE: Backup lock

 TABLE_SCHEMA: 

   TABLE_NAME: 

*************************** 2. row ***************************

    THREAD_ID: 4

    LOCK_MODE: MDL_SHARED_WRITE

LOCK_DURATION: NULL

    LOCK_TYPE: Table metadata lock

 TABLE_SCHEMA: mysql

   TABLE_NAME: event

2 rows in set (0.000 sec)

mysql [localhost:10504] {msandbox} ((none)) > SELECT * FROM performance_schema.metadata_locks\G

*************************** 1. row ***************************

          OBJECT_TYPE: TABLE

        OBJECT_SCHEMA: mysql

          OBJECT_NAME: event

OBJECT_INSTANCE_BEGIN: 139879098948320

            LOCK_TYPE: SHARED_WRITE

        LOCK_DURATION: TRANSACTION

          LOCK_STATUS: GRANTED

               SOURCE: 

      OWNER_THREAD_ID: 28

       OWNER_EVENT_ID: 1

*************************** 2. row ***************************

          OBJECT_TYPE: BACKUP

        OBJECT_SCHEMA: NULL

          OBJECT_NAME: NULL

OBJECT_INSTANCE_BEGIN: 139879098948400

            LOCK_TYPE: BACKUP_SYS_DML

        LOCK_DURATION: STATEMENT

          LOCK_STATUS: GRANTED

               SOURCE: 

      OWNER_THREAD_ID: 28

       OWNER_EVENT_ID: 1

*************************** 3. row ***************************

          OBJECT_TYPE: TABLE

        OBJECT_SCHEMA: performance_schema

          OBJECT_NAME: metadata_locks

OBJECT_INSTANCE_BEGIN: 139878900194512

            LOCK_TYPE: SHARED_READ

        LOCK_DURATION: TRANSACTION

          LOCK_STATUS: GRANTED

               SOURCE: 

      OWNER_THREAD_ID: 29

       OWNER_EVENT_ID: 1

3 rows in set (0.000 sec)

mysql [localhost:10504] {msandbox} ((none)) > SELECT OBJECT_TYPE, OBJECT_SCHEMA, OBJECT_NAME, LOCK_TYPE, LOCK_STATUS, THREAD_ID, PROCESSLIST_ID, PROCESSLIST_INFO FROM performance_schema.metadata_locks INNER JOIN performance_schema.threads ON THREAD_ID = OWNER_THREAD_ID WHERE PROCESSLIST_ID <> CONNECTION_ID()\G

*************************** 1. row ***************************

     OBJECT_TYPE: TABLE

   OBJECT_SCHEMA: mysql

     OBJECT_NAME: event

       LOCK_TYPE: SHARED_WRITE

     LOCK_STATUS: GRANTED

       THREAD_ID: 28

  PROCESSLIST_ID: 4

PROCESSLIST_INFO: NULL

*************************** 2. row ***************************

     OBJECT_TYPE: BACKUP

   OBJECT_SCHEMA: NULL

     OBJECT_NAME: NULL

       LOCK_TYPE: BACKUP_SYS_DML

     LOCK_STATUS: GRANTED

       THREAD_ID: 28

  PROCESSLIST_ID: 4

PROCESSLIST_INFO: NULL

2 rows in set (0.000 sec)

mysql [localhost:10504] {msandbox} ((none)) > SET GLOBAL read_only = ON;

<hanged>

mysql [localhost:10504] {msandbox} ((none)) > show full processlist\G

*************************** 1. row ***************************

      Id: 4

    User: event_scheduler

    Host: localhost

      db: NULL

 Command: Daemon

    Time: 3835

   State: Waiting for next activation

    Info: NULL

Progress: 0.000

*************************** 2. row ***************************

      Id: 37

    User: msandbox

    Host: localhost

      db: NULL

 Command: Query

    Time: 18

   State: Waiting for backup lock

    Info: SET GLOBAL read_only = ON

Progress: 0.000

*************************** 3. row ***************************

      Id: 39

    User: msandbox

    Host: localhost

      db: NULL

 Command: Query

    Time: 0

   State: starting

    Info: show full processlist

Progress: 0.000

3 rows in set (0.000 sec)