When you break Nether Wart, it only drops one Wart regardless of whether it is mature or not. This makes farming Nether Wart impossible as you can't propagate your supply.
NOTE: This is a texture issue. Nether wart appears full grown when it is not. The textures are off by one step, so wait for the wart to grow back to a seedling and then harvest. See
MCPE-14766 for related bug.