Backfocusing in Nikon 14-24/2.8

I've owned the 14-24/2.8 for a couple years and occasionally get good shots out of it, but fairly consistently I've been getting backfocusing with it. I blamed myself at first, and occasionally would get great sharp images, but in retrospect my sharp images where usually objects with some depth where you wouldn't notice a slight back focusing. With people, where precise focusing is more important, the problem is much more consistent. I don't get this at all with my 50/1.8D.  My body is a D90.

I tried to set up a focus test sheet but had a hard time getting a clear answer from this, but with real-life pictures the effect is clear. I sent the lens into Nikon, and it was supposedly tested and calibrated, but the issue remains.

Any ideas for why this would be happening and what can be done about it? I know some cameras have a fine-tuning feature which might help in this circumstance, but unfortunately the D90 does not have this.