For words that only have three letters, there isn't a 4th letter. Such as with the word “increase”, there won't be any additional letters Later on and thus has to be the word "add". The word “addict” is inside the BIP39 wordlist however you would have utilized “addi” for “addict”. https://bookmarknap.com/story9202581/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting