When mining magma blocks in an oceanic trench (that are not surrounded by water and only have water on top) lava does not become obsidian.
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DestructivForce
Affects 18w09a
Fixed in latest 18w10d
From what I can tell, when you break the magma, the bubble columns turn into regular water, but don't update and flow down. If you use a water bucket in the hole caused, the lava turns into obsidian.