Actually finding a number of other rifts where this ^ exact problem is also happening... the last orb of volatile magic is so close to the "endpoint" that the end registers you interacting with it BEFORE you can reach the last VM orb so you cannot complete the rift - no award of mistborn mote, and NO advancement of the achievement. So far I have found 4 of these. Clearly its a consistent problem with the way the rifts are propagating.