Possible the world of relays has many mysteries. To of my friends follow each other on nostr. They use the same clients and they're using (mostly) the same relays. They can see each other fine across the nostr network but they cannot see each other in the zap.stream chat and this has been a thing for like a year. They do see each other in chat on other apps. we've done tons of troubleshooting over time with no joy. It's just the way it is.