Logstash MongoDB Output Plugin - command insert requires authentication

It pretty much has to be, but where to start looking? If you look at the issues for the plugin it comes up from time to time. Some of them appear to be version dependencies where an underlying library change their interfaces.

Here is an example from last year that turned out to be in the mongodb client code.

Last year another user had a similar issue. I pointed them at an unmerged PR and they said they would try to build it themselves. No indication whether that worked.