For words that only have three letters, there is not any 4th letter. For example with the word “increase”, there aren't any additional letters Later on and so must be the word "include". The word “addict” is within the BIP39 wordlist however you might have applied “addi” for “addict”. Multiple https://bookmarklayer.com/story19634638/the-basic-principles-of-bip39-passphrase-generator