Sorry to hear you are having issues. Not happening on my setup. I have  monitor setup, with different wallpaper on each monitor. I have no such issue as you can see from my S/short below.
Would like to know more on your system.
1) Windows version. Please use Winver.exe for full window version numbers.
2) Fences version.
3) Monitor resolution on both. And scaling applied to each monitor.
Stardock Community Assistant.