I don't think that's what it is. There are other places also in the SE's where, when digitally recompositing the original effects, they forgot to include every element. There are missing lights on matte paintings in both SW and ESB, and a missing lightsaber flash frame when Luke cuts himself free of the ice cave, to name a few that I can remember off the top of my head.

Maybe they couldn't find every element and figured no one would notice, but it's equally likely they didn't notice these things themselves. Either way, the redone versions don't entirely do justice to the original work, even if they do technically improve on it in some ways.