UI not visible on third display

Hi, I'm developing for a 3-screen setup in Windows 10. I'm activating the three displays, using a camera for each, and it works great! Except, no UI elements are showing up on the third screen. They *do* show up in the Unity Editor on the "Display 3" game window, but in builds which depend on the screens themselves, the UI isn't visible. I *am* seeing UI elements on both other displays, as expected.

I'm using Screen Space Overlay, Target Display 3 (same target display as that screen's...

UI not visible on third display