I don't get OOF shots unless I've had a brain fart. Just this afternoon, I was taking a few shots of one of our cats with fill flash at f/1.2 and f/1.8. When I was comparing the two shots in LR, I was surprised that the f/1.2 was sharper. I understand that the 50L can be softer at f/1.2, but based on my testing, technique may have something to do with it.
It's a poor craftsman who blames his tools.
Except, in this case, it's the tool's fault. If you look closely, you'll find that your f/1.8 OOF shots are a bit back-focused. The 50L exhibits noticeable focus shift - as you stop down, the focal plane changes. So, since lenses are focused wide open, if you shoot wide open, no problem. By f/4 or so (depending on subject distance), the DoF is getting deep enough to mask the effect. But, between f/1.3 and ~f/4, the focus shift results in back-focusing.
There are some workarounds - shoot wide open or narrower than f/4, MF with the DoF Preview button pressed and a precision focusing screen (problematic on the 7D since you can't easily switch screens), Live View, focus on something a little in front of where you need to, or tweak the focus manually after AF (which takes a lot of practice to become proficient).
Enjoy the lens!