MongoDB credentials are not working with StatefulSet - mongodb

Question

I have this sts:
apiVersion: "apps/v1"
kind: "StatefulSet"
metadata:
name: "mongo-benchmark"
spec:
serviceName: mongo-benchmark-headless
replicas: 1
selector:
matchLabels:
app: "mongo-benchmark"
template:
metadata:
labels:
app: "mongo-benchmark"
spec:
containers:
- name: "mongo-benchmark"
image: "mongo:5"
imagePullPolicy: "IfNotPresent"
env:
- name: "MONGO_INITDB_ROOT_USERNAME"
value: "admin"
- name: "MONGO_INITDB_ROOT_PASSWORD"
value: "admin"
ports:
- containerPort: 27017
name: "mongo-port"
volumeMounts:
- name: "mongo-benchmark-data"
mountPath: "/data/db"
volumes:
- name: "mongo-benchmark-data"
persistentVolumeClaim:
claimName: "mongo-benchmark-pvc"
Everything is deployed.
The root user's username and password is admin
But when I go to the pod terminal and execute these commands I get:
$ mongo
$ use admin
$ db.auth("admin", "admin")
Error: Authentication failed.
0
I can't even read/write from/to other databases.
For example:
$ mongo
$ use test
$ db.col.findOne({})
uncaught exception: Error: error: {
"ok" : 0,
"errmsg" : "not authorized on test to execute command { find: \"col\", filter: {}, limit: 1.0, singleBatch: true, lsid: { id: UUID(\"30788b3e-48f0-4ff0-aaec-f17e20c67bde\") }, $db: \"test\" }",
"code" : 13,
"codeName" : "Unauthorized"
}
I don't know where I'm doing wrong. Anyone knows how to authenticate?