this has happened to me twice. Both times, it turned out that the offending block was, somehow, unstable (red) and was destroying itself after a fairly long delay. Something in the computations determined that one was bad (I had like 5 more blocks high on everything around them, in both cases). Try embedding a larger block into its neighbors to improve support when you fix it. That has worked for me on these. It seems like a rare bug, possibly some sort of floating point roundoff or similar numerical issue that caused an incorrect stability calculation.