Piston is not powered by redstone on an upside down slab

Piston Slab Bug in Minecraft PE
This is a bug I found while trying to make a redstone contraption. When you have a slab, with the redstone dust, going into the piston, the piston doesn't extend. But when you have a normal block it extends. I'm pretty sure it's supposed to extend. Thanks.
Video: https://youtu.be/-T6UXI1zIQI