The Smoke from a campfire with slime blocks behind it makes the color go off

The Smoke from a campfire with slime blocks behind it makes the color go off

What I expected to happen was...:

the color would not change

What actually happened was...:

the color of the smoke changed and went behind the blocks of slime

Steps to Reproduce:
1. place slime blocks behind a campfire
2. look at the slime blocks and the smoke