A quick one in case anyone else hits the same problem as I encountered. The documentation for authentication_ldap_sasl_log_status and authentication_ldap_simple_log_status states these variables can be set to a value between 1 and 5 to control the types of messages logged.
If you set them and still find nothing is logged then sett log_error_verbosity to it’s maximum value of 3 and you should find the messages are output to the error log as expected.
Thats all!