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.
MC-126297
Lava does not always turn down to obsidian when mining magma block
Resolved
Fixed
0
karstvgl
(Unassigned)
Community Consensus
Minecraft 18w08b, Minecraft 18w09a, Minecraft 18w10b
Minecraft 18w10d
Retrieved
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.