IPv6 instance group backend showing as unhealthy

[mountaincode2]

I am following the guidance in:

https://cloud.google.com/load-balancing/docs/internal/setting-up-internal#configure-ipv6-only

I am creating an internal passthrough network load balancer with IPv6 only subnets and backends. I have created the firewall to allow checks from source IPv6 ranges: 2600:2d00:1:b029::/64

Why is my IPv6 instance group showing as unhealthy: