Desktop Video Source Not Working for Secondary Screen

I have 2 displays connected to my Mac. When I select my secondary display as the desktop video source, I get a static charcoal feed and not what is on the display. Selecting my primary display as my desktop video source works, as does selecting all displays (it shows both primary and secondary views side by side).

This is clearly a bug because ManyCam can definitely see what is on both displays… it just does not show the contents of the second display when you select it as a desktop video source on its own.

I need this bug fixed urgently because I use the contents of my second display as a backdrop (virtual background) during my streams and meetings.

EDIT: I am on MacOS Mojave (10.14.6)