The IPv6 address listed would be the outgoing IP address that your ACME client used to connect to the Let’s Encrypt API server.
They wouldn’t know what the associated IPv4 address is - there’s no natural relationship between IPv4 and IPv6 addresses.
Does the IPv6 subnet give you any clue about where it might be hosted? Narrow it down to a specific ISP, datacentre, etc?
Edit: some similar feedback has already been submitted relating to difficulty attributing IPv6 addresses to individual machines - you might want to leave your vote there as well.