Seconds_Behind_Master is always 0

Configuration is the same as in MDEV-9595.

SHOW SLAVE '<any channel>' STATUS\G always shows Seconds_Behind_Master: 0, while SQL and IO thread are running, and Exec_Master_Log_Pos and Relay_Log_Space are changing. I tried stopping SQL thread for a day and stopping both IO and SQL threads for a hour, but when I start the threads again, Seconds_Behind_Master is still 0 all the time. Masters are quite busy servers, and we can observe lags in other slaves running MariaDB 10.0.17.