Win10 Fence transparency shows other wallpaper on multimon setup

Running Win10 anniversary update (1607): In a multimonitor configuration (for me, SurfaceBook + external 4k display), I often see a different blurred image under my fences than the current desktop wallpaper. This seems to be the different wallpaper that is on my secondary display or a prior wallpaper (haven't figured out the pattern).

If I right click the desktop and select next image, it is fixed, but eventually comes back. I've tried the 3.03 beta and still see this.