You should generate 7 bits of the 12th word, and then put them after the bits for your first 11 words. Take note of the bit string at this point.
For reference, generating first 7 bits means you could control range of words you wish to use. It's closest thing to select your own 12 words.
Even using several online tools (such as
https://emn178.github.io/online-tools/sha256.html with input type HEX), SHA256 of
a89ec4e8327577411a104fbecbbf6ac6 is
bf4b881634e88ff59c29caa582413ee050bb1ce9a72272cebe9491f28e474e03It's probably related with big/small endian