For words that only have 3 letters, there isn't any 4th letter. Such as with the word “insert”, there isn't any a lot more letters afterwards and thus must be the word "include". The word “addict” is during the BIP39 wordlist but you would've made use of “addi” for “addict”. https://thegreatbookmark.com/story19029223/invalid-bip39-phrase-things-to-know-before-you-buy