For words that have only three letters, there is not any 4th letter. Such as with the word “insert”, there won't be any more letters Later on and so need to be the word "insert". The word “addict” is from the BIP39 wordlist however , you might have applied “addi” https://sparxsocial.com/story9475391/top-guidelines-of-bip39-word-list-pdf