Troubleshooting x509 unverified signature

I’ve followed the instructions in Sign commits and tags with X.509 certificates | GitLab to setup x509 commit signing, but my signature is not being verified. I’ve added the issuer certificate as per SSL Configuration | GitLab. I’ve run the Cryptographic verification checks but the signature.__send__(:p7).verify still fails. I see the link to the issuer certificate and the identifiers match.

What can I try to verify this commit against this issuer certificate outside of GitLab? I would like to see more troubleshooting information on why this isn’t validating.