So I got a little deeper into this today. Normally I'll leave things so my Image Review will immediately show the screen with all the shot EXIF, small preview and histogram. Today, when I experienced the issue, I just had it set to show me the basic preview screen and instead of showing the file information at the bottom I got a gray line. After zooming in and back out the file information appeared. The image review still remained on until I reset it but after that it was fine. No write/card active light was lit.
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I'll be contacting Nikon and letting them know, and send them this information. Not sure if it will do anything, but it's another piece of the puzzle.
On my D500 this grey bar appears when you touch and hold the bottom of the screen. You can then slide your finger left and right and scroll through images. The moment you lift your finger the bar disappears. It seems that your screen thinks something is touching its bottom (err.. sorry) all the time. Could it be your screen protector messing up with the screen?