some newer networks, like libera.chat, use sha512 for cert fingerprints.
soju seems to only display sha1 and sha256:
certfp fingerprint <network name>
Show SHA-1 and SHA-256 fingerprints for the certificate currently used with the network.
Would you be willing to send a patch?
Done in 12545c07ef4d ("service: show SHA-512 fingerprint").