mycnf under etc does not take effect

Question

1. Install mysql offline.

2. Create /etc/my.cnf file manually.

3. Add some options in /etc/my.cnf.

4. Restart MySQL.

5. Login MySQL, options added does not take effect.

6. /etc/mysql/mysql.conf.d/mysqld.cnf is ok.

OS: Ubuntu 18.04, Ubuntu 20.04 MySQL: 8.0.27

1. There is no specified .cnf file.

2. [mysqld --verbose --help | grep -A 1 'Default options'] result is ok. Default options are read from the following files in the given order: /etc/my.cnf /etc/mysql/my.cnf ~/.my.cnf

3. /etc/my.cnf should be ok, but it's not.