Ok, I read through some forum posts, and apparently Plex does attempt to connect to PubSubServers with IPv4 literals. This is configured in Preferences.xml. For me this server is 184.105.148.115 (SJC) and after changing it to 64:ff9b::b869:9473 (depending on your NAT64 prefix) it worked!
So it’s incorrect that you only need to “have connectivity to plex.tv” since in this case IP literals are used directly. It would be very nice if Plex could simply try to connect using a hostname like sjc.pubsub.plex.tv then all of this could be avoided as DNS64 will give you a reachable IP address if the host is IPv4-only.
