I certainly seemed to remember this too, but there's really no way to confirm it now. It does appear that Luke missing was not scripted. However, if you watch the sequence carefully (as it is now), it seems like it takes Luke forever to unreel the rope from his utility belt, and he is just about ready when there's a cut to the stormtroopers on the other side of the chasm, then when we cut back to Luke he is unreeling the rope again.