I have just tried removing each configuration from the agent, and in the end just started using the vanilla agent, no luck again. This is what it boils down to, and the log kept appearing every minute on every new agent configuration.
With the default Debian 11 /etc/pam.d configuration, these are the 3 lines I get:
May 25 11:33:24 hostname su: (to root) root on none
May 25 11:33:24 hostname su: pam_unix(su:session): session opened for user root(uid=0) by (uid=0)
May 25 11:33:24 hostname su: pam_unix(su:session): session closed for user root