... Wither skulls summoned using commands drop all blocks it destroys.
Test command:
/summon minecraft:wither_skull ~ ~10 ~ {direction:[0.0,-1.0,0.0]}
Go in a world, run the command and if the skull destroys any blocks, it will drop that/those blocks certainly.
I think this should be marked as WAI. The odds of a block dropping when exploded is 1 divided by the explosion power. All wither skulls have a power of 1, so any blocks they destroy should have a 100% chance of dropping resources, regardless of whether or not they were summoned by commands.