For words that only have 3 letters, there is no 4th letter. Such as with the word “insert”, there are no more letters Later on and therefore need to be the word "insert". The word “addict” is while in the BIP39 wordlist however you might have employed “addi” for “addict”. https://total-bookmark.com/story18909270/bip39-seed-phrase-mnemonics-generator-an-overview