I checked the settings, and for "tab AF-4", the selection is "ON".
What you say is exactly what I experience: the camera does not even attempt to focus.
However, with the 24mm f/1.4L II, I seem to be unable to replicate the "issue".
I'm shooting into an almost completely dark closet, ISO 100, f/1.4, 1/60 in order to get the AF to stop working (for diagnostic purposes/to see if I can replicate the issue).
In actual/normal use, I came across this problem when shooting an evening shot (with the 50L). The lens drive just stopped. It had never happened before, so I got curious about it. I switch to MF when necessary, but had never come across the sudden "hang" of the AF lens drive.