There's an issue where zaps to npub.cash addresses don't show on posts in Damus, although they show in some other clients. The same thing happens when Damus users zap Primal addresses. Not sure if it's the same issue, but nostr:npub1vp8fdcyejd4pqjyrjk9sgz68vuhq7pyvnzk8j0ehlljvwgp8n6eqsrnpsw added it to the open issue in GitHub yesterday.