For example, it's possible that none of the first 16 words can be used as the 12th word.
I won’t believe your claim until you provide the proof. I’ve tested dozens of valid BIP39 seed phrases, and this method has consistently worked.

So, please provide the valid 12 word seed and an example set of 16 consecutive words from the BIP39 word list such that none of those 16 words can satisfy the checksum requirement as the 12th word for the first 11 words of that seed.