When Dash Attack it will not destroying anything.
I've tried when was knockback doesn't it was nothing. 

Legacy Console Edition can does destroying anything much as block when something knockback and Dash Attack.

I did set to turn on Mob Griefing it's doesn't work when wither dash attack.

Same as MCPE-39773 I've tested it's doesn't break any ground or cave.

Reproduction Steps

1. Build a wither with soul sand and wither skull make it like T-shape and place skull heads for three or simply summon command.
2. Attack until half health. 
3. Wait for Dash Attack if it does work destroy blocks
4. Hit with knockback enchantment if it destroy blocks.
5. Change mob griefing to false and true.