Running mysql client command inside MySQL pod in Kubernetes raises error

Question

I deployed a StatefulSet on Kubernetes (K3S actually). I used the following manifest:

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: mysql
  namespace: dev
  labels:
    app: myapp
spec:
  selector:
    matchLabels:
      app: myapp
  serviceName: database-svc
  replicas: 1
  template:
    metadata:
      labels:
        app: myapp
    spec:
      containers:
        - name: mysql-db
          image: mysql:8.0.30-debian
          ports:
            - name: tcp
              protocol: TCP
              containerPort: 3306
          env:
            - name: MYSQL_ROOT_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: mysql-secret
                  key: ROOT_PASSWORD
          volumeMounts:
            - name: mysql-data
              mountPath: /var/lib/mysql
  volumeClaimTemplates:
    - metadata:
        name: mysql-data
      spec:
        accessModes:
          - ReadWriteOnce
        resources:
          requests:
            storage: 1Gi

The other referenced resources exist (Service and Secret) and the pod is created correctly. But then I try to run the command kubectl exec -n dev -it mysql-0 -- mysql -u root -p, I insert the correct password and I get the error

ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: YES)
command terminated with exit code 1

If I try to sh into the container and echo the MYSQL_ROOT_PASSWORD variable I get the password that I inserted without success.

What am I missing? How should I connect to the pod from command line?