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 regular camera). All three canvases for the three screens have the exact same values for everything, including the RectTransform and its scale of (0,0,0). One of the things I'm trying to show on all three screens is just a basic Pane stretched to fill the screen, with the Source Image set to the built-in Unity "Background" (I'm using this to fade screens to black and back).

There is one special thing about this screen: the other two screens show up in Windows and Unity with a resolution of 3840x2160, but the third screen only shows up as 1920x1080, possibly a limitation of the GPU (I wouldn't think that would be a problem, but I've never seen this issue before).

Note, the Layer used-to be "Default", I changed it to "UI" in one of the experiments to fix this issue (didn't help)

 

Managed to fix Windows to have all three monitors at the same resolution - and that fixed the UI problem!