My multi-monitor support is broken. How can I fix this?

I am developing a game with multi-monitor support. Previously this feature has been working fine. However, within the past week, it has changed. Now, the “Unity Secondary Display” window opens on the primary monitor on top of the main game window. Furthermore, once you alt-tab to switch into the main game window, the second display camera is drawing in the first display window on top of the main display cameras (at the resolution of the secondary monitor, no less).

This is extremely baffling and confusing. I’ve tried rolling my project back to any and all known working versions - including the very first commit that implemented the feature, at which point I definitely know it was working - and the problem is still occurring. I’ve tried deleting all of the cached files and re-importing/re-compiling everything from scratch but to no avail. And I am encountering this problem on both 5.3.3p1 and 5.4.0b9.

Please help. I’m at my wit’s end.

EDIT: I’ve went ahead and created a diagram just to ensure that I am explaining everything correctly and clearly.

Do you use the Taret Display option of cameras?
Try launching your application with the -multidisplay option:

build_unity_player,exe -multidisplay