Jenkins inconsistency (file changes everytime) - jenkins

Question

I am new to Jenkins and still trying to understand how it actually works.
What I am trying to do is pretty simple. I trigger the build whenever I push it to my Github repo.
Then, I try to ssh into a server.
My pipeline looks like this:
pipeline {
agent any
stages {
stage('SSH into the server') {
steps {
withCredentials([sshUserPrivateKey(
credentialsId: '<id>',
keyFileVariable: 'KEY_FILE')]) {
sh '''
cd ~/.ssh
ls
cat ${KEY_FILE} > ./deployer_key.key
eval $(ssh-agent -s)
chmod 600 ./deployer_key.key
ssh-add ./deployer_key.key
ssh root#<my-server> ps -a
ssh-agent -k
'''
}
}
}
}
}
It's literally a simple ssh task.
However, I am getting inconsistent results.
When I check the log,
Failed Case
Masking supported pattern matches of $KEY_FILE
[Pipeline] {
[Pipeline] sh
+ cd /bms/home/pdsint/.ssh
+ ls
authorized_keys
known_hosts
known_hosts.old
+ cat ****
++ ssh-agent -s
+ eval 'SSH_AUTH_SOCK=/tmp/ssh-hb6yX48CJPQA/agent.51702;' export 'SSH_AUTH_SOCK;' 'SSH_AGENT_PID=51703;' export 'SSH_AGENT_PID;' echo Agent pid '51703;'
++ SSH_AUTH_SOCK=/tmp/ssh-hb6yX48CJPQA/agent.51702
++ export SSH_AUTH_SOCK
++ SSH_AGENT_PID=51703
++ export SSH_AGENT_PID
++ echo Agent pid 51703
Agent pid 51703
+ chmod 600 ./deployer_key.key
+ ssh-add ./deployer_key.key
Identity added: ./deployer_key.key (./deployer_key.key)
+ ssh root#<my-server> docker ps -a
Host key verification failed.
When I ls inside the .ssh directory, it has those files.
In the success case,
Success Case
+ cd /bms/home/pdsint/.ssh
+ ls
authorized_keys
authorized_keys.bak <----------
known_hosts
known_hosts.old
+ cat ****
++ ssh-agent -s
+ eval 'SSH_AUTH_SOCK=/tmp/ssh-yDNVe51565/agent.51565;' export 'SSH_AUTH_SOCK;' 'SSH_AGENT_PID=51566;' export 'SSH_AGENT_PID;' echo Agent pid '51566;'
++ SSH_AUTH_SOCK=/tmp/ssh-yDNVe51565/agent.51565
++ export SSH_AUTH_SOCK
++ SSH_AGENT_PID=51566
++ export SSH_AGENT_PID
++ echo Agent pid 51566
Agent pid 51566
+ chmod 600 ./deployer_key.key
+ ssh-add ./deployer_key.key
Identity added: ./deployer_key.key (./deployer_key.key)
+ ssh root#<my-server> docker ps -a
Warning: Permanently added '<my-server>' (RSA) to the list of known hosts.
It has the authorized_keys.bak file.
I don't really think that file makes the difference, but all success logs have that file while all failure logs do not. Also, I really don't get why each build has different files in it. Aren't they supposed to be independent of each other? Isn't that the point of Jenkins (trying to build/test/deploy in a new environment)?
Any help would be appreciated. Thanks.