slave doesn't report Seconds_Behind_Master when running slave_parallel_workers > 0

Reported here too http://bugs.mysql.com/bug.php?id=84415

When running mysql with slave_parallel_workers > 0 , Seconds_Behind_Master from SHOW SLAVE STATUS doesn't report replication lag.

From SHOW PROCESSLIST, Id 3 is executing an event from 76 seconds ago

slave1 [localhost] {msandbox} ((none)) > SHOW PROCESSLIST;
+----+-------------+-----------+------+---------+------+---------------------------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-------------+-----------+------+---------+------+---------------------------------------------+------------------+
| 1 | system user | | NULL | Connect | 274 | Waiting for master to send event | NULL |
| 2 | system user | | NULL | Connect | 11 | Waiting for dependent transaction to commit | NULL |
| 4 | system user | | NULL | Connect | 76 | Executing event | NULL |
| 5 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 6 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 7 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 8 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 9 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 10 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 11 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 12 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 13 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 14 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 15 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 16 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 17 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 18 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 19 | system user | | NULL | Connect | 274 | Waiting for an event from Coordinator | NULL |
| 20 | msandbox | localhost | NULL | Query | 0 | starting | SHOW PROCESSLIST |
+----+-------------+-----------+------+---------+------+---------------------------------------------+------------------+
19 rows in set (0.00 sec)

But SHOW SLAVE STATUS doesn't report any lag:
slave1 [localhost] {msandbox} ((none)) > SHOW SLAVE STATUS\G
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: 127.0.0.1
                  Master_User: rsandbox
                  Master_Port: 28890
                Connect_Retry: 60
              Master_Log_File: mysql-bin.000001
          Read_Master_Log_Pos: 569563421
               Relay_Log_File: mysql-relay.000004
                Relay_Log_Pos: 284857134
        Relay_Master_Log_File: mysql-bin.000001
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB:
          Replicate_Ignore_DB:
           Replicate_Do_Table:
       Replicate_Ignore_Table:
      Replicate_Wild_Do_Table:
  Replicate_Wild_Ignore_Table:
                   Last_Errno: 0
                   Last_Error:
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 284861060
              Relay_Log_Space: 569564180
              Until_Condition: None
               Until_Log_File:
                Until_Log_Pos: 0
           Master_SSL_Allowed: No
           Master_SSL_CA_File:
           Master_SSL_CA_Path:
              Master_SSL_Cert:
            Master_SSL_Cipher:
               Master_SSL_Key:
        Seconds_Behind_Master: 0
Master_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
                Last_IO_Error:
               Last_SQL_Errno: 0
               Last_SQL_Error:
  Replicate_Ignore_Server_Ids:
             Master_Server_Id: 1
                  Master_UUID: 00028890-1111-1111-1111-111111111111
             Master_Info_File: /home/marcelo.altmann/sandboxes/rsandbox_5_7_15/node1/data/master.info
                    SQL_Delay: 0
          SQL_Remaining_Delay: NULL
      Slave_SQL_Running_State: Waiting for dependent transaction to commit
           Master_Retry_Count: 86400
                  Master_Bind:
      Last_IO_Error_Timestamp:
     Last_SQL_Error_Timestamp:
               Master_SSL_Crl:
           Master_SSL_Crlpath:
           Retrieved_Gtid_Set:
            Executed_Gtid_Set:
                Auto_Position: 0
         Replicate_Rewrite_DB:
                 Channel_Name:
           Master_TLS_Version:
1 row in set (0.00 sec)

Same happens with slave_parallel_type=DATABASE .

How to repeat:
1) Start slave with slave_parallel_workers > 0

2) Create a table with some rows on master:
sysbench --test=/usr/share/sysbench/tests/db/oltp.lua --mysql-host=localhost --mysql-user=msandbox --mysql-password=msandbox --mysql-socket=/tmp/mysql_sandbox28890.sock --mysql-db=test --oltp-table-size=1500000 prepare

3) Delete a big amount of rows in your master:
DELETE FROM sbtest1 WHERE id > 100;

4) Compare output of SHOW PROCESSLIST and SHOW SLAVE STATUS\G