Pistons powered by non-adjacent block

I was building a button panel with three adjacent buttons. In order to hide the wiring, I placed three sticky pistons underneath the blocks with buttons, with the intention that only the piston under the block with the pressed button would activate (pushing a block on top of a redstone torch to send a signal - but that part's not important here). I designed this under 1.4.7 and just tried it under 1.5, only to discover either two or all three of the pistons activated when I pressed a button.

What I expected to happen
When I press a button, only the one piston /under/ the block with the pressed button would trigger. (see second attached image). This is the behaviour I experienced in minecraft version 1.4.7

What actually happened
When I press a button, the correct piston fires, but so do any of its immediate neighbours. (see first attached image).

Steps to reproduce problem

• Place three blocks in a row, and buttons on each of them.
• Place three pistons on top of the blocks, and another three pistons underneath the blocks.
• Press a button.

In both images, the middle (wooden) button has been pressed, yet in the first image you can see the problem behaviour exhibited in minecraft 1.5.

This may be related to MC-108, however unlike that report, this appears to be an issue which has appeared sometime between 1.4.7 and 1.5.