The blocks above 3 pistons next together disappear.

I built a redstone elevator in 1.5.1 but when it updated it wouldn't work, when I tried to fix it I discovered that the blocks above the pistons were disipering.

to reperdoce:
1. place three pistons facing up next to eachother.
2. place a block (not transparent) on each.
3. place a redstone repeater facing towards a block on a piston and power it with a button.
4. place redstone behind each block on each piston.
5. click button.