I believe they are suffering from this bug https://github.com/LemmyNet/lemmy/issues/3198. Several communities have complained about the same thing. There's an issue when a community has moderators from other instances, it makes lemmy think it isn't local to any instance.
That's what I've been trying to do all this time. Usually searching for !community@instance indexes the community for the instance I'm searching on but in this case it isn't working and seems to be a bug like @[email protected] mentioned in their comment already.
I noticed when I search for !community@instance, it looks like nothing pulls up, then I change from 'all' to anything else and it loads enough for me to pull it up and subscribe.
Using the URL to pull them up was only working half the time, but then searching and changing the filter seemed to pick up that slack.