Ah ha -- no, it's the authority_signing_key that I said was only for v3 auths. And I still think that's true.
But the ed25519_signing_cert file is for every relay. It has nothing to do with your bridge authority status in particular.
You have a relay whose signing key (for the relay, not for any authority thing) has just expired. I guess that means you made your relay identity key in an offline way? You need to do whatever the process is for generating a new signing cert from your ed25519_signing_secret_key.