The switch will always try to authenticate AD credentials as the ACS is still up. The fallback for AAA on the IOS will be triggered only when the ACS (in this specific scenario) is down. At that point the switch will get a timeout and move to the "local" IOS database as fallback.
You can configure the AAA command with "local" in front of "group tacacs+" as follows:
aaa authentication login default local group tacacs+
The above command will allow you to authenticate on the switch with both Local IOS credentials and TACACS+ credentials.
For your simulated downtime the IOS will not fallback to the local credentials as the ACS is still able to reply with a Reject to the switch even when the AD is down.
The suggested command will allow you to access the IOS with Local or TACACS+ credentials.
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