You can try using an app manifest for BGInfo and set the following attributes.
<dpiAwareness xmlns="http://schemas.microsoft.com/SMI/2016/WindowsSettings">PerMonitorV2</dpiAwareness>
But again, BGInfo is quite old; newer versions might be available with multi-monitor support. As a sidebar, you could consider alternative methods of setting up desktop wallpapers.
Assign unique desktop backgrounds to each screen based on screen resolution or orientation.
In the days of working from home and hot desking, employees are always on the move. Desktop backgrounds/wallpaper configuration has to change automatically when employees move from one station to another or the display configuration changes.
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