For words that only have 3 letters, there isn't any 4th letter. For example with the word “increase”, there aren't any extra letters Later on and thus have to be the word "insert". The word “addict” is during the BIP39 wordlist however, you might have applied “addi” for “addict”. The https://bookmarksea.com/story19579378/the-basic-principles-of-bip39-passphrase-generator