Mongoose keeps trying to authenticate with wrong admin db even though it is specified - mongodb

Question

The initial connection to the database seems to authenticate fine with this:
mongoose.connect('mongodb://user:pass#localhost/traderdb/?authSource=admin', {
auth: {
authdb: "admin"
}
}, function(e) {
//other things
});
After that however, when I try to create a new session and authenticate the user everything crashes and in the mongo logs I can see that the application tried to authenticate with mongodb using the traderdb database and not the admin database, even though I have authSource set correctly and it connects fine the first time with the same URI.
This is where I initialize the session store. This is placed inside the callback of the mongoose.connect call.
app.use(session({
secret: 'secrettexthere',
saveUninitialized: true,
resave: true,
store: new MongoStore({
url: 'mongodb://user:pass#localhost/traderdb/?authSource=admin',
db: 'traderdb',
collection: 'sessions',
auto_reconnect: true
})
}));
Nothing fails on application startup, and the mongo logs confirm that a connection was successfully made and authenticated with the admin database. After that however, when I try to actually use the session store the application crashes with the error Error: Error setting TTL index on collection : sessions. And in the mongo server logs I can see that my application tried connecting and authenticating with traderdb instead of admin as the authentication source.