Servers under libre.moe operate in a dual-stack, meaning the have both IPv4 and IPv6 networking. However, not all services are reachable by both protocol versions.
All HTTP/HTTPS traffic is handeled via nginx installed on the host and operates on the IPv4 and ::1 IPv6 address of the server. All services that are based purely on web traffic have DNS entries that return both addresses.
Traffic that is directly flowing to a port other than 80 or 443, for example with Mumble using port 64738 for the custom Mumble VoIP protocol, is passed through directly to the software. Due to the fact that I sandbox hosted services from the host, this passthrough sends the traffic to the internal network for hosted software, which is only operates on IPv4.
Therefore, software using non web traffic via other ports have those ports only exposed on the IPv4 address of the host. To avoid issues, the DNS entries for those services are usually adapted to only return the IPv4 address.