This IS a duplicate article, but I want to make it very clear.

Bug Explanation:

This is a bug involving the Magma Block retaining light properties when near an alternate light source. (e.i. Torch, sea lantern, etc.)

This is actually a programmable light source. Let me go in-depth (Warning: Confusing!):

If a light source is near the block, it will retain the lighting that it has. For example, I place a lantern near the magma block, and the light level of the magma block is 6. When I remove the light source, the magma block's light level will remain at 6. Likewise if the light was closer, and stronger, the light level of the magma block after the light source is removed will become stronger.

One thing to point out is that the strength of the light from the magma block can only get stronger, and never dimmer.

In-Game Fix:

Break/replace Magma Block