For words that only have three letters, there is not any 4th letter. For instance with the word “increase”, there are no much more letters Later on and thus has to be the word "insert". The word “addict” is while in the BIP39 wordlist however you might have employed “addi” https://allkindsofsocial.com/story4119283/how-much-you-need-to-expect-you-ll-pay-for-a-good-valid-bip39-seed-phrase