For words that have only three letters, there's no 4th letter. By way of example with the word “insert”, there are no extra letters Later on and thus should be the word "insert". The word “addict” is while in the BIP39 wordlist but you might have utilised “addi” for “addict”. https://bookmarklinking.com/story4972719/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator