17-40L owners: backfocusing at close distances?

Yesterday while doing some close work for the first time with my 17-40L I realized I was getting backfocused shots on some of them.

My test: 17-40L on a 5D, tripod mounted. Positioned camera close to MFD from subject (about 1-2 feet). Set at 40mm f/4, One Shot AF, timer with mirror lockup. Took test shots, both racking the focus ring to the MFD side and the infinity side before starting AF. The weird thing is, all shots starting at infinity are perfectly in focus, while the shots starting at the MFD are backfocused by the same amount.

I thought this was crazy. I searched the web and only found a mention in Ken Rockwell's review where said his copy also backfocuses at 40mm near MFD.

Would anyone be willing to confirm whether their 17-40L backfocuses in this situation?