For words that only have three letters, there isn't any 4th letter. For example with the word “insert”, there aren't any much more letters Later on and thus need to be the word "include". The word “addict” is within the BIP39 wordlist but you might have utilized “addi” for “addict”. https://nybookmark.com/story20512022/the-basic-principles-of-bip39-passphrase-generator