I have had this. Its caused by stability: for some reason (yes, I know!) that one block is not getting the support it needs. You need to raise the ground below it or add iron supports inside it or the like to extent support so it stops breaking. And yes, Its weird at times... I had one 1x1 block that would not endure no matter what I tried (ended up making it a window since it was appropriate height) even though I had a full level of stone including ceiling above that block's height, something was off about that one computation of support.

If you can demostrate it, you may be able to highlight the bug (I do believe its a bad support calculation, round off or some edge case). But its hard to prove.

This is rare. I have only seen it twice, once above, and another later that I fixed with more supports. And I have built a LOT of stone.