Lightroom :: 4 Causes Softness On Corner Of Image

 I am duplicating a painting using my Canon 5Dmk2 and some studio lights. I mounted the 22x28 inch painted canvas to a flat wall. I then mounted my camera on a tripod, perpendicular to the print and the same exact centerline. I used two soft boxes on opposite sides of the painting for even, non-reflective lighting. Then, I metered with a flash meter and shot the painting at 1/160s, f11, ISO 100. I got a perfect exposure. I even used Live View on my camera zoomed in tight to get a super accurate focus.
 
When I bring the image into LR4 (or LR3), the lower left hand corner was quite soft, but the rest of the image was tack sharp. I figured I screwed something up so I reshot the image. I remeasured everything to make sure all was flast, square, true, etc. Then, I shot the image with two very sharp lenses—Canon 100mm f2.8 macro, and Canon 70-200mm f4L lenses, both set at 100mm, f11, 1/160s, ISO 100.
 
When looking at the image on my camera LCD, even zoomed in to max close-up, the entire image is sharp. When I import it into LR, that corner goes soft again! At first it displays sharp, but as soon as the software zooms in, it goes soft and stays soft. I tried enabling and disabling Lens Correction, but that lower left hand corner remains soft no matter what I do. I really don't think it is the lens, camera alignment or the exposure, since it displays sharply on the camera's LCD when zoomed in tight. Both these lenses are very sharp, and besides, I left some headroom on all four edges to avoid possible softness or vignetting.