Put water into a 1*1*1 block (in other words, water cannot flow out of the source block), and then put lava within the same block. The water block changes into lava block instead of obsidian.
MC-129434
Lava substitutes water instead of changing it to obsidian when poured down to a water source block trapped in a 1*1*1 block space
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Minecraft 18w19b, Minecraft 1.13-pre2, Minecraft 1.13-pre5, Minecraft 1.13.1, Minecraft 1.13.2, ..., Minecraft 19w11b, Minecraft 19w12b, Minecraft 1.14, 1.14.4, 1.15 Pre-release 4
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Sorry for creating a duplicate. But so far I don't understand why is it intended? Shouldn't water/lava become obsidian all the time when lava/water is directly poured onto it? Why is it not happening when water/lava is a single source block? In my opinion there's no reasonable account for it.