I dug out a small cube of space (and closed myself in) and put down a Magma block. It produced a little light. I broke the block, and, letting sunlight come in, placed the block back down. I then closed back the hole and the magma block was exerting a light level of (what I believe is) 15, as opposed to if it wasn't exposed.
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[media][media]
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I'm pretty sure that's an intended feature of magma blocks, not a bug.
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Your version is outdated and this is a duplicate of MC-102162.
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@unknown unless you can give the source from mojang stating this, we will not resolve this as intended.