I just made two more test shots and manipulated EXIF data (including the max aperture value as well as the actual aperture setting) in one while leaving the other alone. Same problem. Again, LightRoom sees the change, but Photo Mechanic doesn't. Another interesting point - I never have this problem with ORF files (from the Olympus E-M1). Also, any files exported as jpgs, regardless of which camera, carry the proper EXIF data.
One more detail which might be relevant:
Before changing the EXIF data, lenstype comes up as "0mm f/0" and focal length is nil. But after changing it with lenstagger/EXIFtool, lenstype becomes "AF 0mm f/0" and focal length correctly registers as "58mm"
Is this a quirk that is peculiar to NEF files?