Craig Masters

This was caused by opening the world in 1.8.x temporarily. Also the cause of the empty chests in your other bug report MC-100660

That's a block update detector, sometimes called a BUD switch. From what I can tell after reading your report and being confused as to what you were talking about, and finally getting to the screenshots, this is working as intended. This has been in the game since pistons were added. Use a slab instead of a full block above it, and that won't happen.