For words that have only 3 letters, there is absolutely no 4th letter. For instance with the word “increase”, there aren't any more letters Later on and thus must be the word "add". The word “addict” is inside the BIP39 wordlist however , you might have employed “addi” for “addict”. https://ztndz.com/story21928558/how-much-you-need-to-expect-you-ll-pay-for-a-good-valid-bip39-seed-phrase