For words that only have three letters, there isn't a 4th letter. Such as with the word “increase”, there won't be any much more letters afterwards and thus needs to be the word "include". The word “addict” is inside the BIP39 wordlist however you would have employed “addi” for “addict”. https://top100bookmark.com/story19640018/ledger-seed-phrase-bip39-no-further-a-mystery