Commit signing with SSH key uses SSH_AUTH_SOCK but ignores IdentityAgent

I use Secretive (https://github.com/maxgoedjen/secretive) to store my SSH keys on macOS. I've configured my ssh_config to use it as the IdentityAgent, and Git can push and pull just fine.

However, it seems that Git ignores IdentityAgent when signing commits, resulting in the following error message:

error: No private key found for public key "foo.pub"? fatal: failed to write commit object

I've worked around this by setting SSH_AUTH_SOCK, but this doesn't feel correct to me. Is this intended behaviour?

Thanks, Justin