For words that only have three letters, there isn't a 4th letter. For instance with the word “insert”, there isn't any extra letters Later on and so should be the word "add". The word “addict” is within the BIP39 wordlist however , you would have applied “addi” for “addict”. In https://getsocialsource.com/story4356432/new-step-by-step-map-for-ledger-seed-phrase-bip39