Nether Wart is only found in the Nether Fortresses. You see a lot of Soul Sand everywhere in the Nether, but no Nether Warts on it. You only see it in the Nether Fortress.
MCPE-9431
Nether Wart Spawns
Resolved
Works As Intended
0
(Unassigned)
Unconfirmed
Phone - Android - Samsung Galaxy S5
5.1.1
0.12.1 Pre-release, 0.12.1 Beta 2
Retrieved
How is that a bug? Just go and find a nether fortress yourself (if they were added).