For words that only have three letters, there isn't a 4th letter. Such as with the word “insert”, there isn't any more letters Later on and so have to be the word "increase". The word “addict” is while in the BIP39 wordlist but you might have utilised “addi” for “addict”. https://pr7bookmark.com/story19214445/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition