scp doesn't trigger initial prompt even though known_hosts is empty or deleted

Question

I have a bash script to copy a file to a remote server using scp and expect. The script is as follows:

#!/usr/bin/expect -f
set port [lindex $argv 0]
set file_to_send [lindex $argv 1]
set login [lindex $argv 2]@[lindex $argv 3]
set username [lindex $argv 2]
set hostname [lindex $argv 3]
set password [lindex $argv 4]
set dest_folder [lindex $argv 5]

spawn scp -P $port $file_to_send $login:$dest_folder
expect {
    # this will loop forever
    # {*fingerprint*}
    # sleep 0.8
    {Are you sure you want to continue connecting (yes/no/\[fingerprint\])?}
    {
        exp_send "yes\r"
        exp_continue
    }
    {password:} {send "$password\r"}    
}
expect eof

I want to get the initial prompt, given when my machine has never connected to the remote server. Thus, I have tried both removing the added key inside known_hosts or that whole file in my machine. However, I cannot get the initial prompt anymore. This doesn't happen on another machine. Either method gives me the initial prompt as expected. Why is this happening? What can I do to get what I want?

The outpout of grep -i StrictHostKeyChecking ~/.ssh/config /etc/ssh/ssh_config on all 3 machines is:

grep: /home/ubuntu/.ssh/config: No such file or directory
/etc/ssh/ssh_config:#   StrictHostKeyChecking ask

Answer 1

Found the culprit! UserKnownHostsFile default values are ~/.ssh/known_hosts and ~/.ssh/known_hosts2. I renamed the original known_hosts into known_hosts2 for comparison/debugging purpose. Renaming it to known_hosts3 triggers the initial prompt again.