Coturn does not perform username& password authentication

Question

I have configured the coturn servers with long term cred (basic authen) in aws autoscaling group and placed it behind an AWS NLB.

Below is my coturn configuration in turnserver.conf

listening-port=3478
external-ip=$MY_AWS_NLB_ENDPOINT
min-port=32355
max-port=65535
fingerprint
lt-cred-mech
user=$MY_USERNAME:$MY_PASSWORD
stale-nonce

no-stdout-log
syslog
no-multicast-peers

no-cli

no-tlsv1
no-tlsv1_1
#no-tlsv1_2

However, when I placed it behind the NLB, the app connects to the NLB and works just fine, except there is no authentication part, even if I configure wrong username password in my app. What is wrong with my coturn config? Thanks everyone.