See the attached screenshot.
I'm not sure whether pcmanfm is supposed to place the icons on both screens but is calculating the coordinates to place them wrong (because of my unusual screen layout), or if it is only supposed to place the icons on my main screen and I'm seeing them twice as a side-effect of my right monitor being wider than my left monitor.
If it is only supposed to place the icons on one screen, there is a more serious bug, because my main ("Primary") monitor is supposed to be my right one, not my left one.
# xrandr -q Screen 0: minimum 8 x 8, current 2720 x 1096, maximum 4096 x 4096 VGA-0 connected 1280x1024+0+0 (normal left inverted right x axis y axis) 376mm x 301mm 1280x1024 60.02*+ 75.02 1152x864 75.00 1024x768 75.03 70.07 60.00 800x600 75.00 72.19 60.32 56.25 640x480 75.00 72.81 59.94 HDMI-0 connected primary 1440x900+1280+196 (normal left inverted right x axis y axis) 408mm x 255mm 1440x900 59.89*+ 1280x1024 75.02 60.02 1280x960 60.00 1152x720 60.00 1024x768 75.03 60.00 800x600 75.00 60.32 640x480 75.00 59.94
Also notice the location of the right-click menu in the screenshot - I clicked just to the left of the Wastebasket on the right-hand monitor. So it seems the coordinates being calculated when I click on the desktop on the right-hand monitor are wrong too (and consistent with the icon placement).