I tried to switch on parallel replication from Amazon Aurora to MariaDB.

[my.cnf]
slave-parallel-threads = 4
slave-parallel-mode = optimistic

'set global .slave_parallel_mode=optimistic;'
As I understand now it doesn't have sense because it's rolling back to serial replication. But it introduces a silent problem with Seconds_behind_master. This value always = 0.

So, probably, would be good to keep this metric even in this unnatural but working mode.