I found out accidentally while trying to test one with my SE/30's new graphics card. One of the sense pins might be dirty or damaged because the computer is trying to feed it a normal landscape 640x480 signal, leading to a distorted and overlapping image. Interesting that it manages to display anything at all though.
So it turns out that the Micron Xceed cards don’t auto sense the monitor at all. Although the card boots up without a driver, it does need a control panel add-on to manually set the kind of monitor that’s connected to the video card.
The portrait display runs at 16 shades of gray, while the internal display reverts to black and white while an external display is connected. Notice though that, not only does classic Mac OS let a window span across displays, but it even automatically uses appropriate window chrome for each display, using the System 7 style color chrome on the left display but using the classic monochrome window controls on the internal display.