Secondary nodes in mysql 8.0.15 innodb cluster are not reading group_replication_member_expel_timeout variable from my.cnf
When using the MySQL Shell and the AdminAPI to set up and manage MySQL InnoDB clusters there's no need, and it's not recommended, to manually change MySQL Server system variables and other configurations. The AdminAPI provides the functionalities to do any required configuration changes.

Regarding your need to change the value of group_replication_member_expel_timeout, what you need to do is as simple as:

cluster.setOption("expelTimeout", 60)


This command ensures the option is set on all the cluster members.

To verify the options being used by your cluster, you can use the following function:

cluster.options()


For more information please consult the InnoDB cluster userguide:

https://dev.mysql.com/doc/refman/8.0/en/mysql-innodb-cluster-working-with-cluster.html#mysql-innodb-cluster-setting-options

And for some examples and further details you can check the blog posts series published during the 8.0.14 release:

https://mysqlserverteam.com/mysql-innodb-cluster-whats-new-in-the-8-0-14-ga-release/

The series includes posts explaining the usage of the 'expelTimeout' option and also how to change cluster options 'live'.

Cheers,

Miguel