Redstone on Top of Pistons

MumboJumbo recently shown that you can place redstone on top of pistons, but when retracting the piston, the redstone breaks. I do not know what the intended way is, whether redstone should be on piston all the time or not at all, but still a bug no matter what happens.

Mumbo's video: https://www.youtube.com/watch?v=7okXYwR4EdU