I think it could be something to do with Windows game optimizations. Had he same issue on one of my laptops.

If you run the game in borderless windowed mode then it can end up being a borderless window inside another borderless window created by Windows which then creates issues and problems.

Where as if you run the game full screen Windows still seems to put it in a borderless window.

I found a few threads about this occurring on steam forums though many were related to different games. But you might have some luck if you go Into the final fantasy xiv folder. Right click the game open it's properties and disable Windows game optimizations.