For words that only have 3 letters, there is not any 4th letter. For example with the word “add”, there are no much more letters Later on and thus needs to be the word "increase". The word “addict” is inside the BIP39 wordlist however , you might have employed “addi” https://bookmarkplaces.com/story19546883/bip39-seed-phrase-mnemonics-generator-an-overview