To reproduce:
fill a hole with water
click on the water with a lava bucket
water will switch to lava
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.)
[media]This happens in creative and survival mode. And it's happened in multiple seeds.
Related issues
duplicates
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There are other issues with bucket emptying. This one might be related to MC-126906?

Confirmed for 18w20a.

Confirmed in 18w20c.

I also reported this issue in MC-129434 and MC-129543. They were resolved as duplicates of MC-8930, that didn't annoy me. What actually annoyed me was that MC-8930 had been resolved as working as intended. I really can't understand why the thing works so. From any sense this doesn't follow the basic rule of reaction of contacted water and lava. I have been exclaiming my discontent and asking about the reason of the resolution all the time, but not a single person has replied me. I don't how why's that so. This is obviously a bug and once again, I really don't understand why MC-8930 was resolved as intended.

Confirmed for 18w22a.

Confirmed for 18w22c.

Well why isn't this duplicating MC-8930?

Because nobody though of that.