Taken at 24mm f/4 without correction.
On the camera you can't disable distortion autocorrection. Thus normally you won't see those corners (5+EV vignetting there).
Your findings are not isolated, Fred Miranda has reported that too.
Does the resolution fall below the basement too?
I was about to ask if that's with distortion correction turned off.
I think it's a compromise. The question is, how looks the end results.
No strange, I have seen 6.33eV result at somewhere.
And what about the resolution?