I find my second monitor does this often too. It does change at various stages in time in the catalogue , after restarts, etc, no idea when or why it doesn't render . Sometimes it is slightly soft, sometimes pixelised like it hasn't been rendered even though it has, and was correct days before. It seems to be the loupe mode at 100% as fit works as expected. I've tried deleting and rebuilding previews but it didn't change anything.