For words that only have 3 letters, there is no 4th letter. For example with the word “insert”, there aren't any much more letters Later on and therefore have to be the word "increase". The word “addict” is in the BIP39 wordlist however you would have utilised “addi” for “addict”. https://socialwebconsult.com/story4222333/fascination-about-invalid-bip39-phrase