It would appear you have never done so. Try it and watch. fig 8 is nothing but a "fancy" EDK with an extra loop. The tension legs enter the knot exactly, perfect match for, the EDK. It flips out just like EDK. It doesn't get stuck, wedged in, because it flips upright and has no flat face, just like EDK.
I have done so many times, but you must be using something else.
Please show a photo (or diagram) of how a figure 8 knot can be pulled over a corner without the knot touching it.
simply pre pull the knot over the edge first.
It doesn't work as well if there are multiple edges all the way down a rappel or if the edge isn't at the top of the rap.