For words that only have 3 letters, there isn't a 4th letter. For instance with the word “add”, there are no extra letters Later on and therefore should be the word "add". The word “addict” is during the BIP39 wordlist however , you might have applied “addi” for “addict”. This https://45listing.com/story21498220/an-unbiased-view-of-bip39-seed-phrase-github