pretty sure it’s a bug...
I play on an ipad, never happened before the update
This happened to me too. ipad mini 2, ios 12
Nether portals are also affected. Particle emitting blocks (nether portals, torches) still emit particles.
I would imagine this bug is caused in attempt to limit the capabilities of the observer block. Because the observer block shouldn't be able to detect redstone updates to the block, the feature of redstone updating blocks was simply removed. However, this also causes errors when updating other items attached to the block, such as, in this case, torches.
also affects ios on 1.13.3