I used One shot to get the focus, then switched to MF so that it wouldn't change.
Ah, that might be a problem. Switching the AF/MF switch on the lens can move you out of position enough to throw focus off at f/1.4. I think it can also slightly shift the lens focus.
I believe your f/1.4 shot is simply OOF. Could be that the lens needs some MA or you shifted things enough to move the focus plane off the subject. My 50 f/1.4 is significantly sharper wide open than what your image is showing.