A parallel replica delays all events that the SQL Thread reads in. With the changes from
MDEV-29639, a replica will update Seconds_Behind_Master on the first user-generated event after idling. This mismatch leads to a parallel replica displaying 0 for Seconds_Behind_Master for the entire duration of a configured MASTER_DELAY if it is delaying an internal event, even if there are user-generated events that the IO thread has seen and written to the relay log. One example would be when delaying the first event after issuing START SLAVE.