I had a bit of time and energy, so I revisited that Ansible project. And I actually made some progress. I think Ansible is just failing to parse IPv6 addresses from an inventory file? At least that's my best guess. I reconfigured the target so I can use IPv4 for now, and that worked. And I guess that's fine. In "prod" (as it were) I would be targeting a URL anyway, rather than a raw IP address.