HEIC image thumbnails don't appear on older iPad
I encountered the same issue with a friend's iPad (running 9.3.5) and iPhone 7 (running 11.1.1). Apparently, the iPad Photo app's thumbnailer is unable to handle HEIF files.

I worked around this by going into the iPhone's Settings > Camera > Formats and selecting Most Compatible. Any pictures taken after that show up properly on the iPad.