I have a 50mm f/1.2 and used to own an F4s, never had any focusing problem with the combo during the times I used them. Though beware, at f/1.2 the focus plane at 1.2 is very much near non-existant on any camera and I've used it on fm2n, f3,f4,f6 and even on d700 so it's not the camera here.

Though on the F6 even though you opened it up all the way to f/1.2, the camera will only show it opened all the way to f/1.4 but rest assured the metering thinks otherwise and calculates in the exposure at f/1.2.