I believe this can happen in one of few ways:
1. Transaction was not a Nostr zap. Only a plain Lightning transaction.
2. Zap was sent to somebody that does not have a name or display name set up on their kind 0 profile.
3. Zap was sent anonymously (i.e. to a random pubkey), which is just a derivative of (2).
We should change (2) and (3) to show the abbreviated pubkey as the display name instead of “Unknown”. I’ll fix it.
