For words that only have 3 letters, there is no 4th letter. For instance with the word “increase”, there are no extra letters Later on and thus needs to be the word "increase". The word “addict” is inside the BIP39 wordlist however , you might have used “addi” for “addict”. https://maroonbookmarks.com/story18755507/fascination-about-invalid-bip39-phrase