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.
Confirmed, the mooshrooms are removed client-side while server-side they weren't sheared. Can happen in creative without the need of slabs when sheared from as far away as possible.