I don't have an answer, but I noticed this last fall with my D810 files. It just started and only on import. This was 5.71 LR.
When I moved to CC, I lost the catalog, well, it got a bit corrupted. When I pulled a back up and finally got that to work, the black bars were gone. I am running win7 64 bit.
My card reader is the Lexar USB 3 reader, model LRW300U Rev 3.
Paul