I have both lenses and I use them both regularly, but the 85 1.2L II is my preferred portrait lens. It is very challenging to use wide open, because of the very shallow DOF. But that is where you get the results you can´t get with any other lens. You really have to think what you want in focus and move yourself or your object to make sure you get it. To get a pair of eyes in focus requires that they are totally parallel to the focus plane. You can get get some really exceptional images with it and it gives the most beautiful bokeh I know.
But if you want the whole face, from nose tip to ear, to be in focus, you need to stop it down and then it, in my view, looses its magic and there are other lens alternatives that are easier to work with.
It may be that you should check the micro adjustment for your 100 f2.8L. It does not have to be much off to give you the problem you are describing.