I notice this as well if a lower contrast subject is causing some focus issues, it is quite inconsistent. I can shoot it several times in a row, and some will be in focus and some will be quite off. I am wondering if MatthewK's fixed copy is doing well in these scenarios.
I got my lens back from Canon for the 2nd time and it is much better now where most of the shots are hitting focus perfectly. But there are still those unexplainable misses. Back off to Canon.
I had quite a similar situation with my 24-70 on the wide end a few years back and it took quite a few trips to get it 100% resolved, and where I didn't have to chimp each shot to see if it is in focus. I won't accept anything less with the 24L.