The public key authentication method has a function where the server and client can handshake if a key will be accepted, without having to actually unlock the key and perform the signature operation. This has the benefit of providing the user with an error message without having to enter the passphrase just to discover they don't have access.
However there is a hidden requirement for this to work, and that is that the private key file also has a corresponding public key file with the name <private key>.pub. We have not documented this requirement so users might easily end up in a situation where this function does not work.
If you are on the machine where you created the private key then this usually works ass ssh-keygen will create the .pub file for you. The problem arises when you've taken (just) your private key with you to a different machine.
Note that the private key file does not have the necessary information, so we cannot change the requirement on a .pub file without losing compatibility with OpenSSH's keys. So what we can do here is document the requirement and perhaps add warnings to the log.