I have exactly the same problem, it seems that the option “support chunk” do nothing, I also activated the flag in the destructible setting called " asset Defined support " (I also tried with the option called"World support" too). I watched all the example of the Destructedmap in the content example, and they only use the world support chunk, with just a radial force in order to create impact, but all the part can be destroyed ( if you increase the radius or move the radial force), and they never use the parameter chunk ).
So the examples don’t help, because they never use the chunk parameters.
It could be nice to have more info about the Fracture tool of Unreal 4, or fix the problem, if it’s a problem.
Personnally I tried a lot of option, and I can’t have the result I want. It seems impossible to disable chunk even by activating the support chunk parameter.