Marc,
I am not able to reproduce this.

Here are my steps:
- installation of mariadb-server-10.1_10.1.26-0+deb9u1;
- load and activation of the audit plugin:
        $ cat /etc/mysql/my.cnf | grep -v ^#
        [mysqld]

        plugin_load=server_audit=server_audit.so
        server_audit_file_path=/var/log/mysql/mariadb_server_audit.log
        server_audit_logging=ON

        [client-server]

        !includedir /etc/mysql/conf.d/
        !includedir /etc/mysql/mariadb.conf.d/
- restart mariadb and check that plugin is loaded and activated;
- upgrade to mariadb-server-10.1_10.1.37-0+deb9u1;
        $ sudo apt-get dist-upgrade
- check that audit plugin is still activated;

Can you give me the content of you /etc/mysql/my.cnf file?
Is it possible that on your system a previous version of mariadb/mysql
was installed (before 10.1.26)?

-- 
Faust

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