MariaDB Slave_Parallel_Mode = optimistic seems to be ignored

MariaDB Master/Slave setup.

I have set slave_parallel_mode to be 'optimistic', but show slave status shows it being 'conservative'

SHOW ALL SLAVES STATUS:

Parallell_Mode = conservative

SHOW VARIABLES LIKE 'slave_parallel_mode' = optimistic

I have restarted the slave database multiple times. I have looked in error logs. I can't find any reason why it's conservative when I set it to be optimistic.

Can it be because I have not enabled GTID?

I have read the documentation 100 times https://mariadb.com/kb/en/mariadb/parallel-replication/ It does not say what more is required than setting slave_parallel_mode = optimistic..

Thank you for any help :)