[tripletao]

Everyone agrees that most of the possible shuffles become impossible when a CSPRNG with 256 bits of state is used. The question is just whether that matters practically. The original author seems to imply it does, but I believe they're mistaken.

Perhaps it would help to think of the randomization in two stages. In the first, we select 2^256 members from the set of all possible permutations. (This happens when we select our CSPRNG algorithm.) In the second, we select a single member from the new set of 2^256. (This happens when we select our seed and run the CSPRNG.) I believe that measurable structure in either selection would imply a practical attack on the cryptographic algorithm used in the CSPRNG, which isn't known to exist for any common such algorithm.

[recursivecaveat]

Yeah, you're discarding almost all permutations, but in an unbiased manner. It seems to imply not only an attack, but that your experimental results rely strongly and precisely on some extremely esoteric (otherwise it would've been found already) property of the randomization algorithm. If you can only detect the effect of television on turkeys when using a PRNG whose output is appropriately likely to have a high dimensional vector space when formated as a binary square matrix then I think you should probably go back to the drawing board.