Yes, I know it's 20% possible that I'm wrong about finding a "bug" here, but I'm not worried about being wrong. I'm not entirely sure what's happening here, but these three statements show a surprising problem when you run them more than once.
Curiously, I found a workaround for this bug which is stranger than the bug itself. If you create an IF statement around the Set Desktop action, and every second time that you call it, you use "Fill Screen" instead of "Fit to Screen", then there is no bug. That is so odd, the only explanation I have is something is wrong inside the "Set Desktop Image" action. It took me several hours to realize my code wasn't defective.
It may be that you are required to have your second monitor be smaller in pixels that your first. I should also note that I was using AirPlay with my Apple TV as my second monitor. Not sure if that's important.