Fallback local account works when remote auth server is UP

I have remote authentication configured and have created a fallback local account.

My goal for local fallback account is twofold:

• WORK when remote auth server is DOWN 
• NOT WORK when remote auth server is UP

The first case works fine but I notice that local account works even when remote auth server is UP, which is not pure fallback. How can I achieve that ?

I have followed K67025432 (Configuring remote authentication fallback on BIG-IP systems)

fallback is configured as follows:

auth source {
    description none
    fallback true
    type tacacs
}

Local account: 

auth user emergency {
    description "remote authentication fallback account"
    encrypted-password $6$ogpR9DMV$ImG9kaaaaaaaaaaaaJ4POotYfu4j6B5HjmJMLD6TYpdVRaNhJICke/
    partition Common
    partition-access {
        all-partitions {
            role admin
        }
    }
    session-limit -1
    shell bash
}

My goal is use case in line 3: