For words that only have three letters, there is not any 4th letter. Such as with the word “insert”, there won't be any far more letters Later on and therefore needs to be the word "insert". The word “addict” is within the BIP39 wordlist but you might have applied “addi” https://exactlybookmarks.com/story19426777/5-simple-techniques-for-bip39-word-list-pdf