When replicated events are from Master unaware of MariaDB GTID their handling by
the Parallel slave misses Seconds_Behind_Master updating.

In the bug condition the Show-Slave-Status' field remains unchanged.

Because in such case event execution is sequential the bug
is fixed with deploying the same logics as in the explicit single-threaded mode
with is to set

  Relay_log_event::last_master_timestamp

member early at the end of event reading from the relay log.