For words that have only 3 letters, there is absolutely no 4th letter. Such as with the word “insert”, there isn't any far more letters afterwards and therefore should be the word "include". The word “addict” is while in the BIP39 wordlist but you would have applied “addi” for “addict”. https://thebookmarkfree.com/story19270560/not-known-factual-statements-about-bip39-explained