When a water bucket is emptied into a lava cauldron, a loud hissing sound plays, smoke is generated and an empty cauldron results. The same happens for when lava is emptied into a water cauldron.

According to the resolution of MC-204118, the fact that this behaviour does not occur in Java Edition is intended, resulting in Bedrock Edition being at fault for having it.

(I am reporting this as a parity issue as despite the fact that this behaviour has existed in Bedrock Edition for several updates, it was only added to Java Edition in the last snapshot, making it a valid parity issue.)