i still can't understand when this happen.
but it already happend 5-6 times in a few hours.

sometimes the "allow xfce to manage the desktop" option in "desktop preferences" go away. (and the terminal loose its trasparent background)
it seems to happen when i interact with the trash or press f5 to show new icons on the desktop (example file just downloaded which doesn't show, but they are there)

to solve i have to manually re-enable the option said above.