Invisible Mooshrooms after shearing them through slabs

I was trying to make a cow breeding machine, but then I tried to shear the mooshrooms. Strangely, it decided to disappear. After I killed all of the cows and heard noises, I realized the mooshrooms were invisible. I relogged, and sure enough, they were still there. It only works when you shear them through a slab.

http://www.youtube.com/watch?v=sK9ZoyY1wCA&feature=youtu.be