What I expected to happen was...:
I placed torches in front of magma blocks so that they would emit light level 13, then removed the torches.

What actually happened was...:
When the chunks were unloaded and then reloaded, the magma blocks stopped emitting light at level 13.

Steps to Reproduce:
1. Place magma block with torch next to it, remove torch
2. Unload chunks by moving away from the area
3. Return to the area and the produce light level by the magma blocks will have changed