For words that only have three letters, there isn't a 4th letter. Such as with the word “insert”, there aren't any extra letters Later on and so need to be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you would've applied “addi” for “addict”. In this https://bookmarkassist.com/story18816860/fascination-about-bip39-english-words