When the server parses my.cnf or other option/configuration files, it interprets the !includedir directive to mean that it should include all .cnf and .ini files in the directory:

It is also possible to include all option files in a directory from another option file. For example, to include all option files in /etc/my.cnf.d/, an option file could contain:

[mariadb]
...
!includedir /etc/my.cnf.d/

The option files within the directory are read in alphabetical order.

All option file names must end in .cnf on Unix-like operating systems. On Windows, all option file names must end in .cnf or .ini.

https://mariadb.com/kb/en/library/configuring-mariadb-with-option-files/#including-option-file-directories

This is not the case with MariaDB Connector/C:

It is also possible to include the default option files in a directory from another option file. For example, to include the default option files in /etc/my.cnf.d/, an option file could contain:

[client-mariadb]
...
!includedir /etc/my.cnf.d/

Unlike with MariaDB server, this directive does not configure MariaDB Connector/C to include all option files ending in .cnf on Unix-like operating systems or all option files ending in .cnf and .ini files on Windows. Instead, it just configures MariaDB Connector/C to include the my.cnf in the given directory, and also the my.ini in the given directory if it's Windows.

https://mariadb.com/kb/en/library/configuring-mariadb-connectorc-with-option-files/#including-option-file-directories

This can be seen here:

https://github.com/MariaDB/mariadb-connector-c/blob/1285dc72a043f09d9a51abcfc3a4fbfb5192067e/libmariadb/ma_default.c#L196

https://github.com/MariaDB/mariadb-connector-c/blob/1285dc72a043f09d9a51abcfc3a4fbfb5192067e/libmariadb/ma_default.c#L328

We might want to change this, so that it reads all .cnf and .ini files in the directory, so that MariaDB Connector/C handles !includedir more like MariaDB Server.