First things first.
While LR does keep some exif, it strips the vital data we need to view to see how your camera is actually set up.
Only way to do this that I know of, is to convert any NEF file with ViewNX(free) or resize any in camera jpg with FSViewer(I think IrfanView may also work).
In the exif data is a host of camera related info, such as how it's set up for focus and so on.
Secondly. If you camera is set up for single point focus it shouldn't be using any other focus point other than the one you've chosen!
Because you are using AF-S mode for focusing, there is no need to lock focus. All you need do is to focus once, once you hear the confirm beep, keeping slight pressure on the release button, recompose and then shoot.
Also! don't assume that because you have locked focus, that you have maintained focus either!
Unless you've set the camera on a
tripod, and using focus and recompose, there is almost certainly a very high probability that you're technique is to blame. Also, (which
lens?) the
lens you are using is almost certain to have an amount of field curvature. That is focusing using the central portion of the len's optical path may not be the same plane of focus as it is at the edges.
I have no idea on how good or bad the
Sigma 30/1.4 is with respect to curvature of field.
Also, as I think I'm understanding it: even tho you are in AF-S focus mode and you have set the camera up to only use a single focus point, the camera still automatically chooses it's own (random) focus point?
If this is correct, then I think you haven't properly set the camera up, even tho it seems to work well in landscape orientation, the camera still has the ability(or capability too) focus consistently in the same spot with auto focusing selection.