Blocks disappearing when connected to sticky pistons

I noticed the bug while making a video. A block of redstone attached to an extended sticky piston disappears as soon as another redstone block is placed adjacent to it.

The bug can be seen in this video http://www.youtube.com/watch?v=9DOt2vSs4xI (skip to around 1:02).

If you want to download the map, just let me know.