Is it supposed to have fallen just as Luke walked in front of it?
Yup, that's what's meant to happen, although hopefully people won't come to the conclusion that it just fell off, because with any luck they won't notice it at all.
It was another example of minimal fix. I could have removed it from the first half of the shot, but this would have introduced a (lesser) continuity problem because it was visible a shot or two before.
David