I have set up in my k8s cluster a MongoDB database with the following configuration:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: mongodb-deployment
  labels:
    app: mongodb
spec:
  replicas: 1
  selector:
    matchLabels:
      app: mongodb
  template:
    metadata:
      labels:
        app: mongodb
    spec:
      containers:
        - name: mongodb
          image: mongo
          ports:
          - containerPort: 27017
          env:
            - name: MONGO_INITDB_ROOT_USERNAME
              valueFrom:
                secretKeyRef:
                  name: db-secret
                  key: mongo-root-username
            - name: MONGO_INITDB_ROOT_PASSWORD
              valueFrom:
                  secretKeyRef:
                    name: db-secret
                    key: mongo-root-password
---
apiVersion: v1
kind: Service
metadata:
  name: mongodb-service
spec:
  selector:
    app: mongodb
  type: LoadBalancer
  ports:
    - protocol: TCP
      port: 27020
      targetPort: 27017
      nodePort: 30010

(the type of my service is LoadBalancer so that I can debug it from outside my cluster).

I have a Node.js app inside the k8s cluster (same namespace) which executes the following code:

mongoose.connect(
  `mongodb://${process.env.MONGODB_USERNAME}:${process.env.PASSWORD}@mongodb-service:27020`,
  {
    useNewUrlParser: true,
    useUnifiedTopology: true,
    connectTimeoutMS: 1000,
  },
  (err) => {
    console.log(err);
  }
);

When I try authenticating with the previous code, mongoose fails to connect and console.log(err) prints an AuthenticationFailed MongoError. If however I remove the credentials from the connection string, mongoose manages to connect to the database (which it shouldn't as I have specified credentials as environment variables in my deployment).

The weirdest part is that if I now try connecting using MongoDBCompass on my machine, it's the opposite (or actually it's the expected behavior): the database refuses the connection without credentials but accepts it with the credentials in the connection string.