I am using Windows for my deployment (not by my choice), so it appears my issue are probably related to that.
I tried to force SCRAM-SHA-1 authorization because I was seeing Sasl mechanisms being used (despite not setting those up), but the nodes still authorize with SHA-256 regardless, so I can’t even rule that out. But I also tried 1/2 a dozen other configuration settings (mongod.cfg and windows security and network) to try to get rid of these messages, and they are present no matter what. I also upgraded to Mongo 6 from 5.0.14 to see if it was a bug or something.
Below is the node auth process I am seeing.
{"t":{"$date":"2023-02-28T17:03:27.528+00:00"},"s":"D3", "c":"ACCESS", "id":5286300, "ctx":"conn33","msg":"Starting authentication step","attr":{"step":"SaslStart"}}
{"t":{"$date":"2023-02-28T17:03:27.529+00:00"},"s":"D3", "c":"ACCESS", "id":5286203, "ctx":"conn33","msg":"Updating user name for session","attr":{"userName":{"user":"","db":"local"},"oldName":{"user":"","db":""}}}
{"t":{"$date":"2023-02-28T17:03:27.529+00:00"},"s":"D3", "c":"ACCESS", "id":5286200, "ctx":"conn33","msg":"Setting mechanism name","attr":{"mechanism":"SCRAM-SHA-256"}}
{"t":{"$date":"2023-02-28T17:03:27.529+00:00"},"s":"D3", "c":"ACCESS", "id":5286304, "ctx":"conn33","msg":"Determined mechanism for authentication"}
{"t":{"$date":"2023-02-28T17:03:27.529+00:00"},"s":"D3", "c":"ACCESS", "id":5286301, "ctx":"conn33","msg":"Finished authentication step","attr":{"step":"SaslStart"}}
{"t":{"$date":"2023-02-28T17:03:27.533+00:00"},"s":"D3", "c":"ACCESS", "id":5286300, "ctx":"conn34","msg":"Starting authentication step","attr":{"step":"SaslContinue"}}
{"t":{"$date":"2023-02-28T17:03:27.533+00:00"},"s":"W", "c":"ACCESS", "id":5626700, "ctx":"conn34","msg":"Client has attempted to reauthenticate as a single user","attr":{"user":{"user":"__system","db":"local"}}}
{"t":{"$date":"2023-02-28T17:03:27.533+00:00"},"s":"I", "c":"ACCESS", "id":20250, "ctx":"conn34","msg":"Authentication succeeded","attr":{"mechanism":"SCRAM-SHA-256","speculative":false,"principalName":"__system","authenticationDatabase":"local","remote":"10.2.1.5:52348","extraInfo":{}}}
{"t":{"$date":"2023-02-28T17:03:27.534+00:00"},"s":"D3", "c":"ACCESS", "id":5286305, "ctx":"conn34","msg":"Marking as cluster member"}
{"t":{"$date":"2023-02-28T17:03:27.534+00:00"},"s":"D3", "c":"ACCESS", "id":5286203, "ctx":"conn34","msg":"Updating user name for session","attr":{"userName":{"user":"__system","db":"local"},"oldName":{"user":"","db":"local"}}}
{"t":{"$date":"2023-02-28T17:03:27.534+00:00"},"s":"D3", "c":"ACCESS", "id":5286306, "ctx":"conn34","msg":"Successfully authenticated","attr":{"client":"10.2.1.5:52348","isSpeculative":false,"isClusterMember":true,"mechanism":"SCRAM-SHA-256","user":"__system","db":"local"}}
{"t":{"$date":"2023-02-28T17:03:27.534+00:00"},"s":"D3", "c":"ACCESS", "id":5286301, "ctx":"conn34","msg":"Finished authentication step","attr":{"step":"SaslContinue"}}