The levitation also does not work inside the circle. As Sildarin approaches the outside of the zone, he continues to try to levitate, but with no effect until he is completely outside the marked patch on the floor.
A close examination of Sildarin's footprints does reveal they are different than those mysterious ones from earlier. While the original footprints terminate in mid-stride, Sildarin's are a bit scuffed where he lifts off, and the demarcation isn't as clear.