read_passphrase being called on SSH key even though SSH key is not protected by password

I have a staging server and a production server, and I run identical Bitbucket Pipelines, where I send some commands over SSH. Unfortunately, my pipeline for the production always fails with:

Host key verification failed.

I've tried _everything_, folder permissions, recreating the keys, nothing works. Finally with adding -v to my ssh call, I think I'm a step closer, but still lost. On my staging server, I see something like this:

debug1: Host '$STAGING_SERVER' is known and matches the RSA host key.	
debug1: Found key in /root/.ssh/known_hosts:4	
debug1: ssh_rsa_verify: signature correct

and the rest of the build follows flawlessly. On my production server, however, I see the following:

debug1: Host '$PRODUCTION_SERVER' is known and matches the RSA host key.
debug1: Found key in /root/.ssh/known_hosts:5
debug1: read_passphrase: can't open /dev/tty: No such device or address
debug1: permanently_drop_suid: 0
ssh_askpass: exec(/usr/bin/ssh-askpass): No such file or directory
Host key verification failed.

So it would look like the key _is_ found on my production server, but for some reason on the production server read_passphrase is being called. I've just created a new id_rsa and id_rsa.pub key, with _no_ password, so why the heck is my production server trying to call read_passphrase? My ssh_config and sshd_config on both servers are identical - checked via diff. Another way of looking at it is that ssh_rsa_verify is called immediately on the staging server, while on the production server read_passphrase is called. Any help here would be greatly appreciated, this is driving me crazy!