Wither will destroy a lot of blocks only on hard difficulty.
This phenomenon could not be confirmed in normal mode.
We confirmed this phenomenon in normal (1.20.31) and preview (1.20.40.23).

How to reproduce
1. Set the difficulty level to Hard.
2. Halve Wither's health as usual.
3. The wither destroys blocks to a certain level or abyss.

In The End, withers fall into the abyss and disappear from the world, but in the Nether and Overworld, they destroy blocks underground.