I was using the latest opt-in Beta version on steam. I'm not using DeskScapes, just plain windows settings, Windows supports spanning wallpapers over multiple monitors by default.
I reinstalled and made a few screenshots and a demonstration video:
I don't know why your versioning isn't consistent because 3.1.2 normally isn't the same as 3.12, but that's what I got.
Screenshot of booth Screens:
Creating a Fence:
Video with different Wallpaper fill methods:
only 2 worked for my current wallpaper, but one of them isn't usable, because its distorted.