It not only creates a bias (which decreases entropy), but its an operation that would easily be implemented faster on an FPGA than on a CPU. I mean, its basically a singular LUT, there's nothing more efficiently done on an FPGA than LUTs.
Basically, I think it actually weakens the FPGA-resistance of Cryptonight.
Fortunately, the main idea of the changes would discourage ASICs being made for Monero. But it doesn't seem to discourage FPGAs, which are just one "resynthesize" away from fixing any code changes. And as long as the community becomes accustomed to regular changes, then any issues in the encryption can be fixed later.
Basically, I think it actually weakens the FPGA-resistance of Cryptonight.
Fortunately, the main idea of the changes would discourage ASICs being made for Monero. But it doesn't seem to discourage FPGAs, which are just one "resynthesize" away from fixing any code changes. And as long as the community becomes accustomed to regular changes, then any issues in the encryption can be fixed later.