A single lava source block can be replaced with water by using water bucket

What I expected to happen was...:
The lava/water should turn into obsidian

What actually happened was...:
The lava source block became water source block , and if I do the opposite , using lava bucket click on a single water source block , it became lava source block

Steps to Reproduce:
1. Dig a 1x1x1 hole
2. Put either lava or water in the hole to create a single source block
3. Use a bucket with different liquid and click directly in the hole , and the original liquid will be replaced with another liquid