In a multi-monitor configuration, the display positioned on the top-most and/or left-most is always treated as the primary display even if a different monitor is set to be the actual primary display
Tested in Xubuntu 22.10 but it occurs in older Xfce OSes as well (Mint 21.0, Xubuntu 22.04, Mint 20.3, etc)
Simply put, in a multi-monitor configuration, the monitor positioned in the top-most and/or left-most is always treated as the primary display (i.e. desktop icons and program windows will appear on it) even if you have a different monitor set as the actual primary display.
(technically you can position one monitor ever so slightly to the right and/or below the other without making the other monitor become the secondary display, but that's besides the point)
This is unfortunate for me since I prefer having my two monitors positioned with the primary on the bottom-right and the secondary on the top-left, using those aforementioned corners of each screen to move my mouse cursor between them.