What operating system are you using? What exact version of Lightroom 6 are you using? Have you updated to Lightroom 6.10? Did this just start happening? I'm using Lightroom CC 2015.10, and I haven't seen this happen. I'm using it on Windows 10.
I have upgraded to 6.10 (assuming that is the most recent upgrade). I use Windows 10 as well. I just bought this version of Lightroom and it is a consistent problem. I shoot with a Nikon D500.