The Campfire particles used to be translucent like Java when they got introduced in the earlier betas, but they are fully opaque now.
Linked issues
Comments 5
I possibly know why, it's because there was a rendering issue with the campfire particles appearing behind most blocks and clouds in beta 1.11.0.1. See MCPE-42343.
They removed it because it rendered weirly. Maybe the new rendering engine will help.