Thanks for the reply.
This is a new occurrence of a bug that has existed already. You can see here: "This game is no longer available to join" - #47 by SunTzu-1296 a topic from September was revived after yesterday’s login issues were resolved.
The root cause is different though. For one, you flat out cannot join cross realm at all anymore as of last night.
This is an issue of being on a different server (even same region) than someone else. Games are not interconnected anymore across servers or regions