Look at MC-8930. Lava/water placed onto a single water/lava source block substitutes the water/lava source block instead of changing it into obsidian. Is this reasonable? It violates the basic rule of how water and lava should react when in contact. Should MC-8930 be resolved as WAI?
I've been questioning this all the time under MC-8930 and both my duplicates (MC-129434 and MC-129543), but none of you has responded. Usually when I have an unreasonable explanation you would rush to correct me, but no one has pointed out where I'm wrong about this particular issue, and you don't reopen MC-8930 either. I don't know why's that so.
Hope that you can carefully consider this again. Thank you!
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But the lava/water is still there when water/lava is being placed! The lava/water shouldn't disappear when water/lava is being placed.

Also why is it happening with a single source block and not happening with water/lava in a larger range?
Again, the single source block is replaced.

OK fine. Now that you say so, I'm gonna put this issue on ice although I still don't fully understand why it is replacing the source block instead of solidifying it; but I'll continue to keep track of things relate to this issue.
It's intended because water and lava cannot react when they are both in a single block space because the game wasn't designed that way (it is still possible to code in). The game instead replaces the lava with water (and vice versa) because blocks being mixed within a single space is not how lava/water interactions work currently. It is not a bug that liquid mixing reactions cannot exist within a single block. That is pretty much all I have to say.

You still say that it's possible to code it in. So why didn't Mojang consider about coding it in? This is a perfectly normal feature.
The water/lava is being replaced, they never come in contact with each other to react.