This echoes some of the problems I have had with Windows 10 x64, 3 monitors, and Deskscapes 8. I reported these issues last year and earlier this year in this forum and via support.
I suspect that it is related to the monitor numbering/matching discrepancy or whatever underlies that change in the OS. For me, video wallpapers work just fine on all monitors, but Dynamic Dreams like PhotoDream will ONLY work if they are located on the leftmost monitor (of the 3) AND that monitor is set at the Primary/Main Display...no matter what "number ID" Windows/Deskscapes assigns to it.
I have juggled monitors (all 30" Dells at 2560x1600, see below), positions, which is primary (left, middle, or right), connections (DVI vs DP), and GPUs (old 295/280, Quadro, GTX 970), with all delivering the same results as described above.
Note that this is clearly an OS vs. Deskscapes issue as I can use whatever monitor I want on the same machine/hardware under a secondary boot partition running W7 x64 and have no problems with whatever Dreams I choose to run in whatever position with whatever monitor selected as Primary/Main.
The good news is that, with this one restriction, Deskscapes runs far smoother on this tri-monitor setup under W10 than it does under W7. There is no comparison. W10 is superior in every way, except for this odd bug.
Now for the REAL fun...here is how each "application" reports the monitor order, Left to Right, with the Primary/Main Display being the leftmost monitor on a 970.
MS OS 10 via "Display options" - 2 1 3
NVidia Control Panel - 3 2 1 as connected in "Change Resolution" but 2 1 3 as reported in "Set up Multiple Displays"
Deskscapes 8 Configuration Tool - 1 3 2
Note that the Nvidia control panel and the OS would NOT match up on Windows 10 before this Anniversary Update (but they do now), so that does seem to be a change (probably at the OS level) in this regard. That may be why I was the only person with this problem before the Update and others are reporting it now?
For completeness, though it seems to make no difference how I connected the monitors or in what order in regards to this odd bug, these are the monitors:
Left Monitor is Dell 3008 connected via DVI
Middle Monitor is Dell 3011 connected via DP
Right Monitor is Dell 3007 connected via DVI (also has DP option, makes no difference to behavior)