So this has happened two or three more times to the same block. It's the only block that this has happened to. On the last occurrence it actually dropped the 6 stone. None of the other occasions resulted in any stone being dropped. The block that is disappearing/destructing is in green stability, The blocks on either side are green, and the row of blocks it is sitting on are in direct terrain contact and are blue. I'm going to try replacing the single 4x2 with a couple of smaller blocks and see what results.