For words that only have three letters, there isn't a 4th letter. For instance with the word “add”, there are no extra letters Later on and so needs to be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you might have employed “addi” for “addict”. Mnemonic https://www.storeboard.com/blogs/beauty-and-fashion/opening-the-globe-of-bip39-a-comprehensive-guide-to-mnemonic-expressions-and-word-checklists/6026188