Unfortunately this was an issue with ZeroSSL certificates.
The quick fix is to delete the directory for your domain that’s stuck from Caddy’s data storage, then restart Caddy. This’ll force it to fetch a new certificate.
Since you’re running Caddy as root, I think it’ll be in /root/.local/share/caddy/certificates/acme.zerossl.com-v2-dv90/your-domain