For words that have only three letters, there is not any 4th letter. Such as with the word “insert”, there won't be any a lot more letters afterwards and so needs to be the word "increase". The word “addict” is within the BIP39 wordlist however you would have used “addi” https://modernbookmarks.com/story18827566/the-2-minute-rule-for-bip39-all-words