Created attachment 5389 the partition list can be seen left to right on the desktop, in thunar, in gparted and in the open file dialog of firefox In devices section of the shortcuts side pane, drive partitions are shown in reverse order. I've seen this behavior in both debian, solydx, xubuntu and linux mint xfce. In solydx and debian, i've also seen that the partition desktop icons are in reverse order as well.
Created attachment 5390 xubuntu's thunar with reserve order
Created attachment 5391 linux mint's thunar with reserve order
It should be noted that the solydx screenshot partition icon order changed after i had installed solydk and you can find the original icon order in another solydx screenshot.
Created attachment 5392 the original desktop icon list
Must have been fixed as it no longer happens in the latest Linux Mint or Xubuntu.