Well, I think that dave is right: As the original Emerald Mine engine lets the boulders disappear in the acid (although invisibly), this is the way it should be -- also in the R'n'D game engine. (Try for yourself in R'n'D by using the Emerald Mine engine in the level editor with the test level from the level sketch above.)
So if this doesn't break any existing levels (which I don't really expect), I think this should indeed be treated as a bug, and should be fixed.