Actually I did not make any changes to postgresql.conf
When studying about this parameter I thought I should check what my current setting(for maintenance_work_mem) is and then I found that it is commented in postgresql.conf while the pg_settings has the value enabled.
maintenance_work_mem isn't a setting that can be completely disabled. It's used for normal maintenance operations like autovaccum and index creation. If not set in postgresql.conf, it has a default value of 64MB in 9.6