Context:
Playing Minecraft, had just opened a new world for the snapshot to explore the new features, as 18w11a had many new additions. Flew around for a minute, waited for the world to load. No significant framerate drops. Went into the shallow water by the Taiga for looking at the new water physics if they changed from previous snapshots. I wanted to see if lava killed coral.

Bug - Event:
I forgot that lava turns into obsidian underwater instantly. I broke the block and rapidly placed more obsidian this way for fun. Then I broke a block and replaced it with lava. The lava source block stayed. I placed lava on the beach, and it flowed over the water for one block, and nothing happened.

Bug - Occurrence:
When I placed the lava block with the bucket, it did not interact with the water beneath whatsoever. I figured it would update when a change occurred. I switched into survival and swam up into the lava source block. I took damage and descended back into the water. I did not die, and neither did the lava.

Possible Reproduction (simplified version of the way I had used):
Place on the beach a lava block (or on the shore of an artificial body of water, can be small.)
Swim down into the water for observation if necessary.
The lava block flows into the water as expected, but the two don't interact.
Also, try the setup that is in the image. (Btw, this sand formation is natural, I didn't make the triangle at all).

Addenda:
1. Am I simply forgetting a basic physics of water & lava? The flowing lava is on top of a water block. Shouldn't it form stone?
2. The image I included has a gold block I placed. Even if the lava is placed at this position, on top of the gold block, the furthest possible place, it doesn't interact. A closer example is up against the water.
3. EDIT: Also, even a source block on top of the water will treat it as some kind of solid block and flow one block. (Is this a separate bug, I don't know?)