When 0.13 was released I had discovered this bug. It prevents all use of the spacebar (except if you begin all typing at a certain place with a space) when writing signs, Seed names, World names, player names, and the chat section. I have only now gotten around to reporting this, and I have no certainty that this bug affects MCPE before 0.13, and it does affect all recent versions (0.14, 0.14.2, etc.). Please find some way to fix this, as I am forced to use underscores!