By default if no auth source is passed, the client will use the database the user is connecting too.
For some cloud providers (like Mongo Atlas), the user must be authenticated against 'admin' database even though they are accessing some other db.
The goal is to have a backwards compatible setting such that if the current users don't have this setting, it will continue to work.
Despite having the authSource=admin property in the URI, the authenticate call seems to ignore that. Open to ideas if there is a better way to achieve this
There are a couple of other PRs related to this: