Dispenser with spawn eggs fails to spawn some entities

I had previosly set up a dispenser filled with zombie spawn eggs pointed out into water blocks at one side of a room and connected to a timing circuit (designed to create and transport the zombies away at a timed rate of release.) As of 1.9-pre1, the zombies briefly spawn (just long enough to hear the initial snarl and see a brief flash of visual) but then immediately disappear

further testing reveals that irrespective of the water blocks, some entities fail to spawn fully – Zombies, Blazes, Skeletons all fail – chickens and sheep are able to spawn initially leading me to think that the 2H entities were having trouble, but villagers and horses spawn with no issue while cave spiders fail.