Hostile mobs don't spawn on pressure plates

I'm working on hostile mob farm designs at the moment. For testing purposes I built a two-layered farm floating at y=190. The top layer is just a spawn pad and dispensers with water buckets to wash the mobs down. The bottom layer is pressure plates powering pistons to push the mobs into the hole. In 1.8.9 both farms work fine. In the snapshots at least since 15w47 the piston based one doesn't work, because no hostile mob ever spawns in there. The light level is zero, so the only difference is the pressure plates on the ground.