Without looking into the details, this is very likely due to these domains being hosted on a service that "implements" EDNS Client Subnet (ECS) but is violating the 6th commandment listed at https://developers.google.com/speed/public-dns/docs/ecs, and doing so in the worst way—not only are they not including a valid IPv6 scope, they are not replying to the DNS request (or perhaps returning NXDOMAIN or an empty response).
As I wrote there, "Not returning a valid IPv6 ECS scope is the most frequent reason authoritative name servers do not get ECS data from Google Public DNS."
In this case, the small number of IPv6 clients for those domains means that infrequent dropped responses from the authoritative name server to IPv6 ECS queries are too few to cause the ECS auto-detection to mark the name servers as not supporting ECS (a small number of dropped responses is normal).
As the number of Google Public DNS users who send queries to the service via IPv6 increases, and more relevantly the subset of those users who query the domains you mention increases, eventually Google Public DNS would stop sending ECS, and it would start working on IPv6 again.
In the short term, you should complain to the websites and the DNS hosting services that they use, but probably you just need to route Google Public DNS queries over IPv4 if you care about these domains.