We're so sorry that you're experiencing downshifting concerns with your new Cruze! I understand how frustrating it can be to have concerns with such a new vehicle. We'd be happy to look into this further for you. If interested, please send us a private message.
40mph downshift from 6th to 5th gear can sometimes be a bit abrupt if you were on the throttle while still slowing down past the shift point. I suspect you dealer might be correct that nothing is wrong. The reason this is the first time you noticed its the first time you were slowing down while also still on the gas pedal.