Breaking placed signs on blocks in specific directions gives different wooden plank particles.
Breaking placed sign (facing North) = Acacia texture.
Breaking placed sign (facing East) = Birch texture.
Breaking placed sign (facing South) = Dark Oak texture.
Breaking placed sign (facing West) = Jungle texture.
Note I've seen this happen earlier in 0.15.0 (perhaps earlier), so this has been around, and it's still happening now.
Duplicate of MCPE-16141 – If you have not, please use the search function in the future, to see if your bug has already been submitted, as currently over 47% of tickets are being closed as duplicate.