Water cannot protect blocks from getting destroyed by the Wither

Water cannot protect blocks from getting destroyed by the initial explosion of the Wither. Even water sources are wiped out, which sounds weird to me.

Steps to reproduce:

1. Make a plate of obsidian blocks.

2. Spread water over it.

3. Spawn the Wither.

What I expected:
Obsidians are guarded by water.

What actually happens:
See the screenshots.