For words that have only 3 letters, there isn't a 4th letter. For example with the word “increase”, there are no far more letters Later on and thus must be the word "insert". The word “addict” is inside the BIP39 wordlist but you would've made use of “addi” for “addict”. https://bookmarkfly.com/story18971139/considerations-to-know-about-bip39-seed-phrase-generator