For words that have only three letters, there isn't a 4th letter. One example is with the word “insert”, there are no a lot more letters Later on and so have to be the word "add". The word “addict” is during the BIP39 wordlist however you might have employed “addi” https://cyberbookmarking.com/story18959497/not-known-facts-about-bip39-english-words