Bug - op fails getting SSH key

The problem is that the output of ssh-keygen -t rsa -N "" -b 2048 -m PEM -f test_private_key is a an older PEM block format called pkcs#1 (apparent from the header "BEGIN RSA PRIVATE KEY" as opposed to simply "BEGIN PRIVATE KEY"). 1Password internally supports pkcs#8 and openSSH.

I created an internal issue for supporting your use case as well, but I make no promises as to when this will hit stable, although I don't expect a lot of CLI releases to pass without this being solved.

In the meantime, as a workaround, you can consider generating an RSA key and storing it in pkcs8 pem format: ssh-keygen -m PKCS8 -t rsa before importing it into 1Password.