To reproduce:

1. fill a hole with water
2. click on the water with a lava bucket
3. water will switch to lava
4. works the other way around too

This only happens when you are clicking the water/lava block directly. If you put them next to each other, so they flow into each other, they make obsidian/cobblestone as per normal.

It is happening with bigger bodies of water/lava too. With a pond of water it's hard to notice, because there is other water next to the lava, so it turns into obsidian fairly fast. But if you try it with a lava pond, this is the result. (The lower left block is the one I clicked.)

This happens in creative and survival mode. And it's happened in multiple seeds.