I think it’s whether the pubkey has a nip05 Alias established or not.
Discussion
#[4]
Oh, event linking isn’t supported yet. See JB’s answer here: https://www.nostr.guru/e/f96d502cbcec8597ccd7bbbffe10995d9e77f55a992a4f48ea807c1a8a8639ee
You need to use an ampersand to link events "&"