Here's what I did instead:
nmcli connection modify "cloud-init eth0" ipv6.method manual
nmcli connection modify "cloud-init eth0" ipv6.address 2604:a880:400:d0::xxxx:yyyy/64
nmcli connection modify "cloud-init eth0" ipv6.gateway 2604:a880:400:d0::1
nmcli connection modify "cloud-init eth0" ipv6.dns "2001:4860:4860::8844 2001:4860:4860::8888"
nmcli connection up "cloud-init eth0"