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Your sample image does not look at all like what I get from my 24-105 when shooting a flat subject parallel to the sensor plane at f/8. There really is not any special corner softness problem with this lens when it is working correctly, especially at the long end. (There can be another corner issue related to vignetting at the wide end and at f/4, but that is a complete different thing.)
You might want to try a very careful test just to confirm or eliminate various explanations. I'd put the camera on the tripod, line it up so that it is pointing straight at something flat like a wall (i.e. - the sensor is parallel to the subject), and make a few shots.
If that resolves the issue, the explanation here could be that you were not lined up with the subject - though I have some doubts about that given that there is softness in your example in more than one corner. Or you could have a mechanical problem with the lens that may require a trip back to Canon service.
I had a similar - though perhaps not identical - problem with my 24-105 a few years back. It began to develop some very serious softness on one side of the frame. I finally sent it in for adjustment/repair and it came back better than new. All lenses can go out of adjustment for various reasons, though I have a theory (though you could call it wild speculation!) that this lens may be a bit more affected by jostling around and, as a result, may go out of adjustment a bit more easily.